Science.gov

Sample records for agn accretion disk

  1. Binary Black Holes, Accretion Disks and Relativistic Jets: Photocenters of Nearby AGN and Quasars

    NASA Technical Reports Server (NTRS)

    Wehrle, Ann E.; Jones, Dayton L.; Meier, David L.; Piner, B. Glenn; Unwin, Stephen C.

    2004-01-01

    One of the most challenging questions in astronomy today is to understand the origin, structure, and evolution of the central engines in the nuclei of quasars and active galaxies (AGNs). The favoured theory involves the activation of relativistic jets from the fueling of a supermassive black hole through an accretion disk. In some AGN an outer optically thick, dusty torus is seen orbiting the black hole system. This torus is probably related to an inner accretion disk - black hole system that forms the actual powerhouse of the AGN. In radio-loud AGN two oppositely-directed radio jets are ejected perpendicular to the torus/disk system. Although there is a wealth of observational data on AGN, some very basic questions have not been definitively answered. The Space Interferometry Mission (SIM) will address the following three key questions about AGN. 1) Does the most compact optical emission from an AGN come from an accretion disk or from a relativistic jet? 2) Does the separation of the radio core and optical photocenter of the quasars used for the reference frame tie, change on the timescales of their photometric variability, or is the separation stable at the level of a few microarcseconds? 3) Do the cores of galaxies harbor binary supermassive black holes remaining from galaxy mergers? It is not known whether such mergers are common, and whether binaries would persist for a significant time.

  2. New Insights on the Accretion Disk-Winds Connection in Radio-Loud AGNs from Suzaku

    NASA Technical Reports Server (NTRS)

    Tombesi, F.; Sambruna, R. M.; Reeves, J. N.; Braito, V.; Cappi, M.; Reynolds, S.; Mushotzky, R. F.

    2011-01-01

    From the spectral analysis of long Suzaku observations of five radio-loud AGNs we have been able to discover the presence of ultra-fast outflows with velocities ,,approx.0.1 c in three of them, namely 3C III, 3C 120 and 3C 390.3. They are consistent with being accretion disk winds/outflows. We also performed a follow-up on 3C III to monitor its outflow on approx.7 days time-scales and detected an anti-correlated variability of a possible relativistic emission line with respect to blue-shifted Fe K features, following a flux increase. This provides the first direct evidence for an accretion disc-wind connection in an AGN. The mass outflow rate of these outflows can be comparable to the accretion rate and their mechanical power can correspond to a significant fraction of the bolometric luminosity and is comparable to their typical jet power. Therefore, they can possibly play a significant role in the expected feedback from AGNs and can give us further clues on the relation between the accretion disk and the formation of winds/jets.

  3. Accretion Disks and the Nature and Origin of AGN Continuum Variability

    NASA Astrophysics Data System (ADS)

    Gaskell, C. M.

    2008-04-01

    Theory and observations of the dominant thermal continuum emission in AGN are examined. After correction for reddening, the steady state AGN optical-UV spectral energy distributions (SEDs) are very similar. The SEDs are dominated energetically by the ``big blue bump'' (BBB), but this bump never shows the nu(+1/3) spectrum predicted for a standard thin accretion disk with a (r-0.75) radial temperature gradient. Instead, the observed optical-UV SED implies a temperature gradient of r(-0.57) independent of the thickness of the disk. This means that there is some flow of heat outwards in the disk. The disk is large and the region emitting the optical continuum is as large as the inner broad-line region (BLR). Because optical variability is seen in all AGN on the light-crossing time of the BLR, variations must propagate close to the speed of light, rather than on dynamical timescales. This argues that the energy-generation mechanism is electromagnetic rather that hydrodynamic. Since the velocities are near the speed of light, there can be significant local anisotropy in the emission. The large rapid variations of the BBB imply that the magnetohydrodynamic energy generation is fundamentally unstable. Because of the inevitable radial temperature gradient in the accreting material, different spectral regions come predominantly from different radii, and variations in different spectral regions correspond to variability at different radii. This explains the frequently observed independence of X-ray and optical variations, cases of variability at lower energies leading variability at higher energies, and rapid changes in emission-line reverberation lags. Some observational tests of the local variability hypothesis are proposed.

  4. The Dynamics and Radiative Properties of Clouds in Magnetized Accretion Disk Outflows in AGNs

    NASA Astrophysics Data System (ADS)

    Contopoulos, J.; Kartje, J. F.; Konigl, A.

    1996-05-01

    We have embarked on a general analysis of multi-component outflows from magnetized accretion disks in AGNs. In this scenario, dense diamagnetic clouds are embedded in a centrifugally driven wind and are uplifted from the disk surface by the dynamical drag of the wind as well as by the ``melon seed'' magnetic pressure gradient force. We follow the motion of the clouds above the disk surface, taking account of the radiation pressure force exerted by the central continuum. We consider both line and continuum radiation and investigate the effects of differing spectral energy distributions and degrees of emission isotropy on the cloud dynamics (comparing, e.g., the isotropic, thermal emission from QSOs with the beamed, nonthermal emission from a relativistic jet in BL Lac objects). We study the evolution of the clouds as they accelerate through the wind, considering possible sources of confinement (including magnetic stresses of the ambient field and hydrodynamic ram pressure), and estimating the cloud lifetimes against disruptive processes such as evaporation and dynamical instabilities. We also evaluate the impact of radiation pressure on the wind itself, via the force on dust grains in the wind and through the momentum transferred by the clouds. We examine the spectral signatures of accelerated clouds in the context of this dynamical scheme. Using the calculated velocity and ionization structure of the clouds throughout the flow, we compute absorption line profiles in the AGN continuum spectrum as a function of viewing angle through the wind. We apply our results to the interpretation of the observed optical/UV spectra from BALQSOs, and of the broad EUV and X-ray absorption features detected in a number of BL Lac objects. We also consider possible applications to other AGNs in which radiative acceleration of clouds has been inferred (e.g., ``warm absorber'' sources).

  5. AGN Unification, X-Ray Absorbers and Accretion Disk MHD Winds

    NASA Technical Reports Server (NTRS)

    Kazanas, Demos

    2011-01-01

    We present the 2D photoionization structure of the MHD winds of AGN accretion disks. We focus our attention on a specific subset of winds, those with poloidal currents that lead to density profiles n(r) \\propto 1/r. We employ the code XSTAR to compute the local ionization balance, emissivities and opacity which are then used in the self-consistent transfer of radiation and ionization of a host of ionic species of a large number of elements over then entire poloidal plane. Particular attention is paid to the Absorption Measure Distribution (AMD), namely their hydrogen-equivalent column of these ions per logarithmic 7 interval, dN_H/dlog ? (? = L/n(r)r(sup 2) is the ionization parameter), which provides a measure of the winds' radial density profiles. For the given density profile, AMD is found to be independent of ?, in good agreement with analyses of Chandra and XMM data, suggesting the specific profile as a fundamental AGN property. Furthermore, the ratio of equatorial to polar column densities of these winds is \\simeq 10(exp 4); as such, it is shown they serve as the "torus" necessary for AGN unification with phenomenology consistent with the observations. The same winds are also shown to reproduce the observed columns and velocities of C IV and Fe XXV of SAL QSOs once the proper ionizing spectra and inclination angles are employed.

  6. Accretion Timescales from Kepler AGN

    NASA Astrophysics Data System (ADS)

    Kasliwal, Vishal P.; Vogeley, Michael S.; Richards, Gordon T.

    2015-01-01

    We constrain AGN accretion disk variability mechanisms using the optical light curves of AGN observed by Kepler. AGN optical fluxes are known to exhibit stochastic variations on timescales of hours, days, months and years. The excellent sampling properties of the original Kepler mission - high S/N ratio (105), short sampling interval (30 minutes), and long sampling duration (~ 3.5 years) - allow for a detailed examination of the differences between the variability processes present in various sub-types of AGN such as Type I and II Seyferts, QSOs, and Blazars. We model the flux data using the Auto-Regressive Moving Average (ARMA) representation from the field of time series analysis. We use the Kalman filter to determine optimal mode parameters and use the Akaike Information Criteria (AIC) to select the optimal model. We find that optical light curves from Kepler AGN cannot be fit by low order statistical models such as the popular AR(1) process or damped random walk. Kepler light curves exhibit complicated power spectra and are better modeled by higher order ARMA processes. We find that Kepler AGN typically exhibit power spectra that change from a bending power law (PSD ~ 1/fa) to a flat power spectrum on timescales in the range of ~ 5 - 100 days consistent with the orbital and thermal timescales of a typical 107 solar mass black hole.

  7. Analytical calculations of the radial structure of self-gravitating and magnetized accretion disks in AGN.

    NASA Astrophysics Data System (ADS)

    Lu, Y.; Yang, L.; Wu, S.

    1997-11-01

    The variation with radius of the physical parameters(scale height, central temperature, and central density) of a self-gravitation and magnetized accretion disk is examined.We have adopted the different opacities in different regions as in the work by Collin-Souffrin and Dumont (1990A&A...229..292C), and we consider a disc model with a 10^7^Msun_ black hole at the center and with an accretion rate of 0.01Msun_/yr. Most of our results are very similar to theirs, but there are some differences between ours and theirs, that is: the critical radius where X-rays are able to heat significantly the disc is not sensitive to self-gravitation. However, self-gravitation has an influence on the location of the radius which emits the bulk of UV radiation, and the magnetic field affects both of them.

  8. Modeling X-ray Absorbers in AGNs with MHD-Driven Accretion-Disk Winds

    NASA Astrophysics Data System (ADS)

    Fukumura, Keigo; Kazanas, D.; Shrader, C. R.; Tombesi, F.; Contopoulos, J.; Behar, E.

    2013-04-01

    We have proposed a systematic view of the observed X-ray absorbers, namely warm absorbers (WAs) in soft X-ray and highly-ionized ultra-fast outflows (UFOs), in the context of magnetically-driven accretion-disk wind models. While potentially complicated by variability and thermal instability in these energetic outflows, in this simplistic model we have calculated 2D kinematic field as well as density and ionization structure of the wind with density profile of 1/r corresponding to a constant column distribution per decade of ionization parameter. In particular we show semi-analytically that the inner layer of the disk-wind manifests itself as the strongly-ionized fast outflows while the outer layer is identified as the moderately-ionized absorbers. The computed characteristics of these two apparently distinct absorbers are consistent with X-ray data (i.e. a factor of ~100 difference in column and ionization parameters as well as low wind velocity vs. near-relativistic flow). With the predicted contour curves for these wind parameters one can constrain allowed regions for the presence of WAs and UFOs.The model further implies that the UFO's gas pressure is comparable to that of the observed radio jet in 3C111 suggesting that the magnetized disk-wind with density profile of 1/r is a viable agent to help sustain such a self-collimated jet at small radii.

  9. Obscured accretion from AGN surveys

    NASA Astrophysics Data System (ADS)

    Vignali, Cristian

    2014-07-01

    Recent models of super-massive black hole (SMBH) and host galaxy joint evolution predict the presence of a key phase where accretion, traced by obscured Active Galactic Nuclei (AGN) emission, is coupled with powerful star formation. Then feedback processes likely self-regulate the SMBH growth and quench the star-formation activity. AGN in this important evolutionary phase have been revealed in the last decade via surveys at different wavelengths. On the one hand, moderate-to-deep X-ray surveys have allowed a systematic search for heavily obscured AGN, up to very high redshifts (z~5). On the other hand, infrared/optical surveys have been invaluable in offering complementary methods to select obscured AGN also in cases where the nuclear X-ray emission below 10 keV is largely hidden to our view. In this review I will present my personal perspective of the field of obscured accretion from AGN surveys.

  10. Revealing the Evolving Accretion Disk Corona in AGNs with Multi-Epoch X-ray Spectroscopy: the case of Mrk 335

    NASA Astrophysics Data System (ADS)

    Ballantyne, David R.; Keek, Laurens

    2016-04-01

    Active galactic nuclei host an accretion disk with an X-ray producing corona around a supermassive black hole. In bright sources, such as the Seyfert 1 galaxy Mrk 335, reflection of the coronal emission off the accretion disk has been observed. Reflection produces numerous spectral features, such as the Fe Kα emission line and absorption edge, which allow various properties of the inner accretion disk and corona to be constrained. We perform a multi-epoch spectral analysis of a dozen XMM-Newton, Suzaku, and NuSTAR observations of Mrk 335, and optimize the fitting procedure to unveil correlations between the Eddington ratio and multiple spectral parameters. We find that the ionization parameter of the accretion disk correlates strongly with the Eddington ratio: the inner disk is more strongly ionized at higher flux. Interestingly, the slope of the correlation is less steep than previously predicted. Furthermore, the cut-off of the power-law spectrum increases in energy with the Eddington ratio, whereas the reflection fraction exhibits a decrease. We interpret this behaviour as geometrical changes of the corona as a function of the accretion rate. Below ~10% of the Eddington limit, the compact and optically thick corona is located close to the inner disk, whereas at higher accretion rates the corona is likely optically thin and extends vertically further away from the disk surface. Compared to previous work that considered individual spectra, we find that multi-epoch spectroscopy is essential for breaking degeneracies in the spectral fits and for obtaining accurate spectral parameters. Furthermore, we show that this method provides a powerful tool to study coronal evolution. The rich archives of XMM-Newton, Suzaku, and NuSTAR provide the opportunity to extend this investigation to include several other bright AGN, which will reveal whether the behaviour that we found is common or unique to Mrk 335.

  11. Slim accretion disks

    SciTech Connect

    Abramowicz, M.A.; Czerny, B.; Lasota, J.P.; Szuszkiewicz, E.

    1988-09-01

    A new branch of equilibrium solutions for stationary accretion disks around black holes is found. These solutions correspond to moderately super-Eddington accretion rates. The existence of the new branch is a consequence of an additional cooling due to general relativistic Roche lobe overflow and horizontal advection of heat. On an accretion rate versus surface density plane the new branch forms, together with the two standard branches (corresponding to the Shakura-Sunyaev accretion disk models) a characteristically S-shaped curve. This could imply a limit cycle-type behavior for black hole accretion flows with accretion rates close ot the Eddington one. 29 references.

  12. Probing AGN Accretion Physics through AGN Variability: Insights from Kepler

    NASA Astrophysics Data System (ADS)

    Kasliwal, Vishal Pramod

    that the DRW is insufficient to characterize AGN variability. We provide a new approach to probing accretion physics with variability by decomposing observed light curves into a set of impulses that drive diffusive processes using C-ARMA models. Applying our approach to Kepler data, we demonstrate how the time-scales reported in the literature can be interpreted in the context of the growth and decay time-scales for flux perturbations and tentatively identify the flux perturbation driving process with accretion disk turbulence on length-scales much longer than the characteristic eddy size. Our analysis technique is applicable to (1) studying the connection between AGN sub-type and variability properties; (2) probing the origins of variability by studying the multi-wavelength behavior of AGN; (3) testing numerical simulations of accretion flows with the goal of creating a library of the variability properties of different accretion mechanisms; (4) hunting for changes in the behavior of the accretion flow by block-analyzing observed light curves; and (5) constraining the sampling requirements of future surveys of AGN variability.

  13. Hot Accretion Disks Revisited

    NASA Astrophysics Data System (ADS)

    Bjoernsson, Gunnlaugur; Abramowicz, Marek A.; Chen, Xingming; Lasota, Jean-Pierre

    1996-08-01

    All previous studies of hot (Tp 1010-1012 K), optically thin accretion disks have neglected either the presence of e+ e- pairs or advective cooling. Thus all hot disk models constructed previously have not been self-consistent. In this paper we calculate local disk models including pair physics, relevant radiative processes in the hot plasma, and the effect of advective cooling. We use a modification of the Björnsson & Svensson mapping method. We find that the role of e+ e- pairs in the structure of hot, optically thin accretion disks is far less significant than was previously thought. The improved description of the radiation-matter interactions provided in the present paper modify the previously obtained values of the critical parameters characterizing advectively dominated flows.

  14. Spherical accretion and AGN feedback

    NASA Astrophysics Data System (ADS)

    Nulsen, Paul

    2014-06-01

    For a supermassive black hole accreting from a hot, quasi-spherical atmosphere, it is almost inevitable that the fluid approximation fails inside some point within the Bondi radius, but well outside the black hole event horizon. Within the region where the particle mean free paths exceed the radius, the flow must be modeled in terms of the Fokker-Planck equation. In the absence of magnetic fields, it is analogous to the "loss cone" problem for consumption of stars by a black hole. The accretion rate is suppressed well below the Bondi accretion rate and a significant power must be conveyed outward for the flow to proceed. This situation is complicated significantly by the presence of a magnetic field, but I will argue that the main outcomes are similar. I will also argue that the power emerging from such a flow, although generally far too little to suppress cooling on large scales, is an important ingredient of the AGN feedback cycle on scales comparable to the Bondi radius.

  15. Accretion disk coronae

    NASA Technical Reports Server (NTRS)

    White, N. E.; Holt, S. S.

    1981-01-01

    Recent observations of partial X-ray eclipses from 4U1822-37 have shown that the central X-ray source in this system is diffused by a large Compton-thick accretion disk corona (ADC). Another binary, 4U2129-47, also displays a partial eclipse and contains an ADC. The possible origin of an ADC is discussed and a simple hydrostatic evaporated ADC model is developed which, when applied to 4U1822-37, 4U2129+47 and Cyg X-3, can explain their temporal and spectral properties. The quasi-sinusoidal modulation of all three sources can be reconciled with the partial occultation of the ADC by a bulge at the edge of the accretion disk which is caused by the inflowing material. The height of this bulge is an order of magnitude larger than the hydrostatic disk height and is the result of turbulence in the outer region of the disk. The spectral properties of all three sources can be understood in terms of Compton scattering of the original source spectrum by the ADC. Spectral variations with epoch in Cyg X-3 are probably caused by changes in the optical depth of the corona. A consequence of our model is that any accreting neutron star X-ray source in a semi-detached binary system which is close to its Eddington limit most likely contains an optically thick ADC.

  16. ACCRETION OUTBURSTS IN CIRCUMPLANETARY DISKS

    SciTech Connect

    Lubow, S. H.; Martin, R. G.

    2012-04-20

    We describe a model for the long-term evolution of a circumplanetary disk that is fed mass from a circumstellar disk and contains regions of low turbulence (dead zones). We show that such disks can be subject to accretion-driven outbursts, analogous to outbursts previously modeled in the context of circumstellar disks to explain FU Ori phenomena. Circumplanetary disks around a proto-Jupiter can undergo outbursts for infall accretion rates onto the disks in the range M-dot{sub infall} approx. 10{sup -9} to 10{sup -7} M{sub Sun} yr{sup -1}, typical of accretion rates in the T Tauri phase. During outbursts, the accretion rate and disk luminosity increases by several orders of magnitude. Most of the planet mass growth during planetary gas accretion may occur via disk outbursts involving gas that is considerably hotter than predicted by steady state models. For low infall accretion rates M-dot{sub infall} {approx}< 10{sup -10} M{sub sun} yr{sup -1} that occur in late stages of disk accretion, disk outbursts are unlikely to occur, even if dead zones are present. Such conditions are favorable for the formation of icy satellites.

  17. A Direct Linkage between AGN Outflows in the Narrow-line Regions and the X-Ray Emission from the Accretion Disks

    NASA Astrophysics Data System (ADS)

    Wang, J.; Xu, D. W.; Wei, J. Y.

    2016-03-01

    The origin of outflow in the narrow-line region (NLR) of the active galactic nucleus (AGN) is studied in this paper by focusing on the relationship between the [O iii]λ5007 line profile and the hard-X-ray (in a bandpass of 2-10 keV) emission from the central super-massive black hole (SMBH) in type-I AGNs. A sample of 47 local X-ray selected type-I AGNs at z\\lt 0.2 is extracted from the 2XMMi/SDSS-DR7 catalog, which was originally cross-matched by Pineau et al. The X-ray luminosities in an energy band from 2 to 10 keV of these luminous AGNs range from 1042 to {10}44 {erg} {{{s}}}-1. A joint spectral analysis is performed on their optical and X-ray spectra, in which the [O iii] line profile is modeled by a sum of several Gaussian functions to quantify its deviation from a pure Gaussian function. The statistics allow us to identify a moderate correlation with a significance level of 2.78σ: luminous AGNs with stronger [O iii] blue asymmetry tend to have steeper hard-X-ray spectra. By identifying the role of L/{L}{Edd} on the correlation at a 2-3σ significance level in both direct and indirect ways, we argue that the photon index versus the asymmetry correlation provides evidence that the AGN’s outflow commonly observed in its NLR is related to the accretion process occurring around the central SMBH, which favors the wind/radiation model as the origin of the outflow in luminous AGNs.

  18. Overview of recent advances in accretion disk theory

    NASA Astrophysics Data System (ADS)

    Ohsuga, Ken

    2012-07-01

    The accretion disk theory, which is initiated in 1970's, has made a success for understanding the powerful compact objects, XRBs, AGNs, and so on. Although one- dimensional accretion disk models (standard disk, slim disk, RIAF) were constructed based on the phenomenological α-viscosity prescription, multi-dimensional MHD/Radiation-MHD simulations are recently performed to resolve the disk structure and dynamics from the first principle. The time variations of the disk and the disk-jet connection are also investigated by the multi-dimensional study. We briefly summarize the disk models and introduce the recent advances of the numerical simulations of the black-hole accretion flows and outflows.

  19. ACCRETING CIRCUMPLANETARY DISKS: OBSERVATIONAL SIGNATURES

    SciTech Connect

    Zhu, Zhaohuan

    2015-01-20

    I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10{sup –10} M {sub ☉} yr{sup –1} around a 1 M{sub J} planet can be brighter than the planet itself. A moderately accreting circumplanetary disk ( M-dot ∼10{sup −8} M{sub ⊙} yr{sup −1}; enough to form a 10 M{sub J} planet within 1 Myr) around a 1 M{sub J} planet has a maximum temperature of ∼2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 M{sub J} planet with a ''hot start''. To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 M{sub J} ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L', M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks ( M-dot ≲10{sup −9} M{sub ⊙} yr{sup −1}) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.

  20. Relativistic particle transport in hot accretion disks

    NASA Technical Reports Server (NTRS)

    Becker, Peter A.; Kafatos, Menas; Maisack, Michael

    1994-01-01

    Accretion disks around rapidly rotating black holes provide one of the few plausible models for the production of intense radiation in Acitve Galactic Nuclei (AGNs) above energies of several hundred MeV. The rapid rotation of the hole increases the binding energy per nucleon in the last stable orbit relative to the Schwarzschild case, and naturally leads to ion temperatures in the range 10(exp 12) - 10(exp 13) K for sub-Eddington accretion rates. The protons in the hot inner region of a steady, two-temperature disk form a reservoir of energy that is sufficient to power the observed Energetic Gamma Ray Experiment Telescope (EGRET) outbursts if the black hole mass is 10(exp 10) solar mass. Moreover, the accretion timescale for the inner region is comparable to the observed transient timescale of approximately 1 week. Hence EGRET outbursts may be driven by instabilities in hot, two-temperature disks around supermassive black holes. In this paper we discuss turbulent (stochastic) acceleration in hot disks as a possible source of GeV particles and radiation. We constrain the model by assuming the turbulence is powered by a collective instability that drains energy from the hot protons. We also provide some ideas concerning new, high-energy Penrose processes that produce GeV emission be directly tapping the rotational energy of Kerr black holes.

  1. RINGED ACCRETION DISKS: EQUILIBRIUM CONFIGURATIONS

    SciTech Connect

    Pugliese, D.; Stuchlík, Z. E-mail: zdenek.stuchlik@physics.cz

    2015-12-15

    We investigate a model of a ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the general relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can then be determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We present also a perturbation analysis based on evolution of the oscillating components of the ringed disk. The dynamics of the unstable phases of the ringed disk evolution seems to be promising in relation to high-energy phenomena demonstrated in active galactic nuclei.

  2. Observational evidence for thin AGN disks

    NASA Technical Reports Server (NTRS)

    Netzer, Hagai

    1992-01-01

    AGN spectrum and spectral features, polarization, inclination, and X-ray line and continuum reflection features are discussed in a critical way in order to determine the ones that are the least model-dependent. The sign and strength of absorption and emission edges are found to be model-dependent, and relativistic broadening and shifting makes them hard to detect. The presence or absence of the predicted Lyman edge polarization feature may be used as a decisive test for thin, bare AGN disks. Other good model-independent tests are several inclination-related line and continuum correlations in big AGN samples. It is shown that electron temperature near the surface of the disk can greatly exceed the disk equilibrium temperature, which causes deviations from LTE. This effect must be incorporated into realistic disk models.

  3. Accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Abramowicz, M. A.

    1994-01-01

    The physics of accretion flow very close to a black hole is dominated by several general relativistic effects. It cannot be described by the standard Shakura Sunyaev model or by its relativistic version developed by Novikov and Thome. The most important of these effects is a dynamical mass loss from the inner edge of the disk (Roche lobe overflow). The relativistic Roche lobe overflow induces a strong advective cooling, which is sufficient to stabilize local, axially symmetric thermal and viscous modes. It also stabilizes the non-axially-symmetric global modes discovered by Papaloizou and Pringle. The Roche lobe overflow, however, destabilizes sufficiently self-gravitating accretion disks with respect to a catastrophic runaway of mass due to minute changes of the gravitational field induced by the changes in the mass and angular momentum of the central black hole. One of the two acoustic modes may become trapped near the inner edge of the disk. All these effects, absent in the standard model, have dramatic implications for time-dependent behavior of the accretion disks around black holes.

  4. Outflows from Accretion Disks around Compact Objects

    NASA Astrophysics Data System (ADS)

    Jiao, Cheng-Liang; Wu, Xue-Bing

    2013-02-01

    We solve the set of hydrodynamic equations for accretion disks in the spherical coordinates (rθφ) to obtain the explicit structure along the θ direction. The results display thinner, quasi-Keplerian disks for Shakura-Sunyaev Disks (SSDs) and thicker, sub-Keplerian disks for Advection Dominated Accretion Flows (ADAFs) and slim disks, which are consistent with previous popular analytical models, while an inflow region and an outflow region always exist, which supports the results of some recent numerical simulation works. Our results indicate that the outflows should be common in various accretion disks and stronger in slim disks and ADAFs.

  5. The Kepler Light Curves of KSwAGS AGN: A Unique Window into Accretion Physics

    NASA Astrophysics Data System (ADS)

    Smith, Krista Lynne; Mushotzky, Richard; Boyd, Padi; Edelson, Rick; Howell, Steve; Gelino, Dawn; Brown, Alex

    2016-08-01

    The Kepler-Swift Active Galaxies Survey (KSwAGS) discovered ~160 AGN in the Kepler and K2 fields. The optical Kepler and K2 light curves of these AGN are by far the most precise and evenly-sampled ever obtained. There are unique challenges involved in adapting Kepler/K2 data for use with AGN since the Kepler pipeline removes stochasticity; however, once mitigated, these data provide an unprecedented glimpse of the accretion disk's variability. We have also conducted follow-up spectral observations to determine black hole masses and accretion rates for the sample, which fill a wide parameter space (6.9 < Log MBH < 9.4, 0.003 < L/Ledd < 0.6). These, in tandem with the light curves, may be able to distinguish between different accretion models.

  6. Foundations of Black Hole Accretion Disk Theory

    NASA Astrophysics Data System (ADS)

    Abramowicz, Marek A.; Fragile, P. Chris

    2013-12-01

    This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).

  7. Magnetic Fields and Outflows from AGN Disks

    NASA Astrophysics Data System (ADS)

    Lovelace, Richard V. E.; Bisnovatyi-Kogan, Gennady S.; Rothstein, D. M.

    2010-11-01

    Activity of the nuclei of galaxies involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (Z) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr (Z) which depends mainly on the midplane thermal to magnetic pressure ratio ˜>1 and the Prandtl number of the turbulence P = viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (˜>1) large-scale field does not di use away as suggested by earlier work.

  8. Why Do T Tauri Disks Accrete?

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee; D'Alessio, Paola; Calvet, Nuria; Muzerolle, James

    2006-01-01

    Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks scales roughly with the square of the central stellar mass. No dependence of accretion rate on stellar mass is predicted by the simplest version of the Gammie layered disk model, in which nonthermal ionization of upper disk layers allows accretion to occur via the magnetorotational instability. We show that a minor modification of Gaminie's model to include heating by irradiation from the central star yields a modest dependence of accretion on the mass of the central star. A purely viscous disk model could provide a strong dependence of accretion rate on stellar mass if the initial disk radius (before much viscous evolution has occurred) has a strong dependence on stellar mass. However, it is far from clear that at least the most massive pre-main-sequence disks can be totally magnetically activated by X-rays or cosmic rays. We suggest that a combination of effects are responsible for the observed dependence, with the lowest mass stars having the lowest mass disks, which can be thoroughly magnetically active, while the higher mass stars have higher mass disks that have layered accret,ion and relatively inactive or "dead" central zones at some radii. In such dead zones, we suggest that gravitational instabilities may play a role in allowing accretion to proceed. In this connection, we emphasize the uncertainty in disk masses derived from dust emission and argue that T Tauri disk masses have been systematically underestimated by conventional analyses. Furtlier study of accretion rates, especially in the lowest mass stars, would help to clarify the mechanisms of accretion in T Tauri stars.

  9. PRIMUS: The Relationship between Star Formation and AGN Accretion

    NASA Astrophysics Data System (ADS)

    Azadi, Mojegan; Aird, James; Coil, Alison L.; Moustakas, John; Mendez, Alexander J.; Blanton, Michael R.; Cool, Richard J.; Eisenstein, Daniel J.; Wong, Kenneth C.; Zhu, Guangtun

    2015-06-01

    We study the evidence for a connection between active galactic nuclei (AGNs) fueling and star formation by investigating the relationship between the X-ray luminosities of AGNs and the star formation rates (SFRs) of their host galaxies. We identify a sample of 309 AGNs with {10}41\\lt {L}X\\lt {10}44 erg s-1 at 0.2\\lt z\\lt 1.2 in the PRIMUS redshift survey. We find AGNs in galaxies with a wide range of SFR at a given LX. We do not find a significant correlation between SFR and the observed instantaneous LX for star-forming AGN host galaxies. However, there is a weak but significant correlation between the mean LX and SFR of detected AGNs in star-forming galaxies, which likely reflects that LX varies on shorter timescales than SFR. We find no correlation between stellar mass and LX within the AGN population. Within both populations of star-forming and quiescent galaxies, we find a similar power-law distribution in the probability of hosting an AGN as a function of specific accretion rate. Furthermore, at a given stellar mass, we find a star-forming galaxy ˜2-3 more likely than a quiescent galaxy to host an AGN of a given specific accretion rate. The probability of a galaxy hosting an AGN is constant across the main sequence of star formation. These results indicate that there is an underlying connection between star formation and the presence of AGNs, but AGNs are often hosted by quiescent galaxies.

  10. Accretion disks in Algols: Progenitors and evolution

    NASA Astrophysics Data System (ADS)

    Van Rensbergen, W.; De Greve, J. P.

    2016-08-01

    Context. There are only a few Algols with measured accretion disk parameters. These measurements provide additional constraints for tracing the origin of individual systems, narrowing down the initial parameter space. Aims: We investigate the origin and evolution of six Algol systems with accretion disks to find the initial parameters and evolutionary constraints for them. Methods: With a modified binary evolution code, series of close binary evolution are calculated to obtain the best match for observed individual systems. Results: Initial parameters for six Algol systems with accretion disks were determined matching both the present system parameters and the observed disk characteristics. Conclusions: When Roche lobe overflow (RLOF) starts during core hydrogen burning of the donor, the disk lifetime was found to be short. The disk luminosity is comparable to the luminosity of the gainer during a large fraction of the disk lifetime.

  11. Coronal accretion: the power of X-ray emission in AGN

    NASA Astrophysics Data System (ADS)

    Liu, B.-F.; Taam, R. E.; Qiao, E.; Yuan, W.

    2016-02-01

    The optical/UV and X-ray emissions in luminous AGN are commonly believed to be produced in an accretion disk and an embedded hot corona respectively. We explore the possibility that a geometrically thick coronal gas flow, which is supplied by gravitational capture of interstellar medium or stellar wind, condenses partially to a geometrically thin cold disk and accretes via a thin disk and a corona onto the supermassive black hole. We found that for mass supply rates less than about 0.01 (expressed in Eddington units), condensation does not occur and the accretion flow takes the form of a corona/ADAF. For higher mass supply rates, corona gas condenses to the disk. As a consequence, the coronal mass flow rate decreases and the cool mass flow rate increases towards the black hole. Here the thin disk is characterized by the condensation rate of hot gas as it flows towards the black hole. With increase of mass supply rate, condensation becomes more efficient, while the mass flow rate of the coronal gas attains values of order 0.02 in the innermost regions of the disk, which can help to elucidate the production of strong X-ray with respect to the optical and ultraviolet radiation in high luminosity AGN.

  12. Generalized Similarity for Accretion/Decretion Disks

    NASA Astrophysics Data System (ADS)

    Rafikov, Roman R.

    2016-10-01

    Decretion (or external) disks are gas disks freely expanding to large radii due to their internal stresses. They are expected to naturally arise in tidal disruption events, around Be stars, in mass-losing post-main-sequence binaries, as a result of supernova fallback, etc. Their evolution is theoretically understood in two regimes: when the central object does not exert torque on the disk (a standard assumption for conventional accretion disks) or when no mass inflow (or outflow) occurs at the disk center. However, many astrophysical objects—circumbinary disks, Be stars, neutron stars accreting in a propeller regime, etc.—feature non-zero torque simultaneously with the non-zero accretion (or ejection of mass) at the disk center. We provide a general description for the evolution of such disks (both linear and nonlinear) in the self-similar regime, to which the disk should asymptotically converge with time. We identify a similarity parameter λ, which is uniquely related to the degree, to which the central mass accretion is suppressed by the non-zero central torque. The known decretion disk solutions correspond to the two discrete values of λ, while our new solutions cover a continuum of its physically allowed values, corresponding to either accretion or mass ejection by the central object. A direct relationship between λ and central \\dot{M} and torque is also established. We describe the time evolution of the various disk characteristics for different λ, and show that the observable properties (spectrum and luminosity evolution) of the decretion disks, in general, are different from the standard accretion disks with no central torque.

  13. A COMMON SOURCE OF ACCRETION DISK TILT

    SciTech Connect

    Montgomery, M. M.; Martin, E. L.

    2010-10-20

    Many different system types retrogradely precess, and retrograde precession could be from a tidal torque by the secondary on a misaligned accretion disk. However, a source that causes and maintains disk tilt is unknown. In this work, we show that accretion disks can tilt due to a force called lift. Lift results from differing gas stream supersonic speeds over and under an accretion disk. Because lift acts at the disk's center of pressure, a torque is applied around a rotation axis passing through the disk's center of mass. The disk responds to lift by pitching around the disk's line of nodes. If the gas stream flow ebbs, then lift also ebbs and the disk attempts to return to its original orientation. To first approximation, lift does not depend on magnetic fields or radiation sources but does depend on the mass and the surface area of the disk. Also, for disk tilt to be initiated, a minimum mass transfer rate must be exceeded. For example, a 10{sup -11} M{sub sun} disk around a 0.8 M{sub sun} compact central object requires a mass transfer rate greater than {approx} 8 x 10{sup -11} M{sub sun} yr{sup -1}, a value well below the known mass transfer rates in cataclysmic variable dwarf novae systems that retrogradely precess and exhibit negative superhumps in their light curves and a value well below mass transfer rates in protostellar-forming systems.

  14. Review of gravitomagnetic acceleration from accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2015-11-01

    We review the development of the equations of gravitoelectromagnetism and summarize how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near the accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism to produce collimated jets, it is a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  15. Gravitomagnetic acceleration from black hole accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2016-05-01

    We demonstrate how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near an accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism contributing to the production of jets, it presents a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  16. Stability of MRI Turbulent Accretion Disks

    NASA Astrophysics Data System (ADS)

    Takahashi, H. R.; Masada, Y.

    2010-12-01

    We study the stability of geometrically thin accretion disks with non-standard α parameter, which characterizes the efficiency of the angular momentum transport. Following recent results of numerical simulations of the Magnetorotational instability (MRI) driven turbulence, we assume that α increases with the magnetic Prandtl number. By adopting Spitzer's microscopic diffusivities, we obtain local structures of geometrically thin accretion disks consistently including effects of MRI-driven turbulence. Since the magnetic Prandtl number increases with the temperature, the efficiency of the angular momentum transport and thus viscous heating rate are smaller for a larger radius when δ > 0. We find that such disks can be unstable to gravitational, thermal, and secular instabilities. It is most remarkable feature that the thermal and secular instabilities can grow in the middle part of accretion disks even when the radiation pressure is negligible, while the standard Shakura & Sunyaev's accretion disk (constant α) is stable to these instabilities. We conclude that it would be difficult to maintain the steady mass accretion state unless the Pm-dependence of the MRI-driven turbulence is weak. Consideration of Pm dependence of α due to the MRI-driven turbulence may make the phase transition of accretion disks less mysterious.

  17. GAS ACCRETION FROM A CIRCUMBINARY DISK

    SciTech Connect

    Hanawa, Tomoyuki; Ochi, Yasuhiro; Ando, Koichi

    2010-01-01

    A new computational scheme is developed to study gas accretion from a circumbinary disk. The scheme decomposes the gas velocity into two components one of which denotes the Keplerian rotation and the other of which does the deviation from it. This scheme enables us to solve the centrifugal balance of a gas disk against gravity with better accuracy, since the former inertia force cancels the gravity. It is applied to circumbinary disk rotating around binary of which primary and secondary has mass ratio, 1.4:0.95. The gravity is reduced artificially softened only in small circular regions around the primary and secondary. The radii are 7% of the binary separation and much smaller than those in the previous grid based simulations. Seven models are constructed to study dependence on the gas temperature and the initial inner radius of the disk. The gas accretion shows both fast and slow time variations while the binary is assumed to have a circular orbit. The time variation is due to oscillation of spiral arms in the circumbinary disk. The masses of primary and secondary disks increase while oscillating appreciably. The mass accretion rate tends to be higher for the primary disk although the secondary disk has a higher accretion rate in certain periods. The accretion rates onto the two components are similar within the fluctuations in late times, i.e., after the binary rotates more than 20 times. The primary disk is perturbed intensely by the impact of gas flow so that the outer part is removed. The secondary disk is quiet in most of time on the contrary. Both the primary and secondary disks have traveling spiral waves which transfer angular momentum within them.

  18. X-ray Spectroscopy of the most extreme Balmer-line disk-emission AGN

    NASA Astrophysics Data System (ADS)

    Eracleous, Michael

    2005-10-01

    We propose to obtain simultaneous X-ray and UV observations of the most extreme AGN with double-peaked Balmer emission lines (FWHM > 19000km/s). We will use the XMM-Newton data to measure their X-ray spectral shapes and construct spectral energy distributions. We will combine these with measurements of the optical emission line profiles (from simultaneous HET observations) and luminosities to (a) test models for illumination of the outer disk by the central X-ray source, (b) test models for the structure of the inner accretion disk (radiatively inefficient accretion vs. standard disk), and (c) compare the X-ray-to-optical properties of these broadest Balmer-line objects to those of broad and narrow line AGN to test models for the origin of the low-ionization broad lines.

  19. Accretion of solid materials onto circumplanetary disks from protoplanetary disks

    SciTech Connect

    Tanigawa, Takayuki; Maruta, Akito; Machida, Masahiro N.

    2014-04-01

    We investigate the accretion of solid materials onto circumplanetary disks from heliocentric orbits rotating in protoplanetary disks, which is a key process for the formation of regular satellite systems. In the late stage of the gas-capturing phase of giant planet formation, the accreting gas from protoplanetary disks forms circumplanetary disks. Since the accretion flow toward the circumplanetary disks affects the particle motion through gas drag force, we use hydrodynamic simulation data for the gas drag term to calculate the motion of solid materials. We consider a wide range of size for the solid particles (10{sup –2}-10{sup 6} m), and find that the accretion efficiency of the solid particles peaks around 10 m sized particles because energy dissipation of drag with circum-planetary disk gas in this size regime is most effective. The efficiency for particles larger than 10 m becomes lower because gas drag becomes less effective. For particles smaller than 10 m, the efficiency is lower because the particles are strongly coupled with the background gas flow, which prevents particles from accretion. We also find that the distance from the planet where the particles are captured by the circumplanetary disks is in a narrow range and well described as a function of the particle size.

  20. Magnetically elevated accretion disks in active galactic nuclei: broad emission line regions and associated star formation

    NASA Astrophysics Data System (ADS)

    Begelman, Mitchell C.; Silk, Joseph

    2016-10-01

    We propose that the accretion disks fueling active galactic nuclei are supported vertically against gravity by a strong toroidal (φ -direction) magnetic field that develops naturally as the result of an accretion disk dynamo. The magnetic pressure elevates most of the gas carrying the accretion flow at R to large heights z ˜ 0.1 R and low densities, while leaving a thin dense layer containing most of the mass - but contributing very little accretion - around the equator. We show that such a disk model leads naturally to the formation of a broad emission line region through thermal instability. Extrapolating to larger radii, we demonstrate that local gravitational instability and associated star formation are strongly suppressed compared to standard disk models for AGN, although star formation in the equatorial zone is predicted for sufficiently high mass supply rates. This new class of accretion disk models thus appears capable of resolving two longstanding puzzles in the theory of AGN fueling: the formation of broad emission line regions and the suppression of fragmentation thought to inhibit accretion at the required rates. We show that the disk of stars that formed in the Galactic Center a few million years ago could have resulted from an episode of magnetically elevated accretion at ˜0.1 of the Eddington limit.

  1. Where a Neutron Star's Accretion Disk Ends

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-03-01

    In X-ray binaries that consist of a neutron star and a companion star, gas funnels from the companion into an accretion disk surrounding the neutron star, spiraling around until it is eventually accreted. How do the powerful magnetic fields threading through the neutron star affect this accretion disk? Recent observations provide evidence that they may push the accretion disk away from the neutron stars surface.Truncated DisksTheoretical models have indicated that neutron star accretion disks may not extend all the way in to the surface of a neutron star, but may instead be truncated at a distance. This prediction has been difficult to test observationally, however, due to the challenge of measuring the location of the inner disk edge in neutron-star X-ray binaries.In a new study, however, a team of scientists led by Ashley King (Einstein Fellow at Stanford University) has managed to measure the location of the inner edge of the disk in Aquila X-1, a neutron-star X-ray binary located 17,000 light-years away.Iron line feature detected by Swift (red) and NuSTAR (black). The symmetry of the line is one of the indicators that the disk is located far from the neutron star; if the inner regions of the disk were close to the neutron star, severe relativistic effects would skew the line to be asymmetric. [King et al. 2016]Measurements from ReflectionsKing and collaborators used observations made by NuSTAR and Swift/XRT both X-ray space observatories of Aquila X-1 during the peak of an X-ray outburst. By observing the reflection of Aquila X-1s emission off of the inner regions of the accretion disk, the authors were able to estimate the location of the inner edge of the disk.The authors find that this inner edge sits at ~15 gravitational radii. Since the neutron stars surface is at ~5 gravitational radii, this means that the accretion disk is truncated far from the stars surface. In spite of this truncation, material still manages to cross the gap and accrete onto the

  2. Accretion disk thermal instability in galactic nuclei

    NASA Astrophysics Data System (ADS)

    Mineshige, S.; Shields, G. A.

    1990-03-01

    The nonlinear evolution and spatial propagation of the thermal instability in accretion disks in galactic nuclei are investigated. Integrations of the vertical structure of the disks are described for different alpha prescriptions, and the thermal stability is examined. Global time-dependent calculations of the unstable disks are performed which show that there are two distinct types of behavior according to the assumed prescription for the viscosity parameter: the 'purr' type and the 'roar' type. The roar type is analyzed in some detail.

  3. The Disk-Jet Connection in Radio-Loud AGN: The X-Ray Perspective

    NASA Technical Reports Server (NTRS)

    Sambruna, Rita

    2008-01-01

    Unification schemes assume that radio-loud active galactic nuclei (AGN) contain an accretion disk and a relativistic jet perpendicular to the disk, and an obscuring molecular torus. The jet dominance decreases with larger viewing angles from blazars to Broad-Line and Narrow-Line Radio Galaxies. A fundamental question is how accretion and ejecta are related. The X-rays provide a convenient window to study these issues, as they originate in the innermost nuclear regions and penetrate large obscuring columns. I review the data, using observations by Chandra but also from other currently operating high-energy experiments. Synergy with the upcoming GLAST mission will also be highlighted.

  4. X-ray Signatures of Accretion in AGNs with Intermediate-mass Black Holes

    NASA Astrophysics Data System (ADS)

    Ho, Luis

    2009-09-01

    Supermassive (10^6-10^9 solar mass) black holes (BHs) are closely linked with the evolution of early-type galaxies. Our group has discovered a new class of AGNs with intermediate-mass (10^4-10^6 solar mass) BHs in late-type galaxies. These objects offer important clues to the nature of the seeds of quasars, and their mergers may produce significant gravity waves. We have started to systematically study their multiwavelength properties. A pilot Chandra program revealed that they are unusually X-ray bright, possibly because their low BH masses and high accretion rates sustain a slim accretion disk. We propose to extend and confirm our preliminary results by performing a comprehensive survey of the X-ray properties of a larger sample of this new class of AGNs.

  5. Earth, Moon, Sun, and CV Accretion Disks

    NASA Astrophysics Data System (ADS)

    Montgomery, M. M.

    2009-11-01

    Net tidal torque by the secondary on a misaligned accretion disk, like the net tidal torque by the Moon and the Sun on the equatorial bulge of the spinning and tilted Earth, is suggested by others to be a source to retrograde precession in non-magnetic, accreting cataclysmic variable (CV) dwarf novae (DN) systems that show negative superhumps in their light curves. We investigate this idea in this work. We generate a generic theoretical expression for retrograde precession in spinning disks that are misaligned with the orbital plane. Our generic theoretical expression matches that which describes the retrograde precession of Earths' equinoxes. By making appropriate assumptions, we reduce our generic theoretical expression to those generated by others, or to those used by others, to describe retrograde precession in protostellar, protoplanetary, X-ray binary, non-magnetic CV DN, quasar, and black hole systems. We find that spinning, tilted CV DN systems cannot be described by a precessing ring or by a precessing rigid disk. We find that differential rotation and effects on the disk by the accretion stream must be addressed. Our analysis indicates that the best description of a retrogradely precessing spinning, tilted, CV DN accretion disk is a differentially rotating, tilted disk with an attached rotating, tilted ring located near the innermost disk annuli. In agreement with the observations and numerical simulations by others, we find that our numerically simulated CV DN accretion disks retrogradely precess as a unit. Our final, reduced expression for retrograde precession agrees well with our numerical simulation results and with selective observational systems that seem to have main-sequence secondaries. Our results suggest that a major source to retrograde precession is tidal torques like that by the Moon and the Sun on the Earth. In addition, these tidal torques should be common to a variety of systems where one member is spinning and tilted, regardless if

  6. EARTH, MOON, SUN, AND CV ACCRETION DISKS

    SciTech Connect

    Montgomery, M. M.

    2009-11-01

    Net tidal torque by the secondary on a misaligned accretion disk, like the net tidal torque by the Moon and the Sun on the equatorial bulge of the spinning and tilted Earth, is suggested by others to be a source to retrograde precession in non-magnetic, accreting cataclysmic variable (CV) dwarf novae (DN) systems that show negative superhumps in their light curves. We investigate this idea in this work. We generate a generic theoretical expression for retrograde precession in spinning disks that are misaligned with the orbital plane. Our generic theoretical expression matches that which describes the retrograde precession of Earths' equinoxes. By making appropriate assumptions, we reduce our generic theoretical expression to those generated by others, or to those used by others, to describe retrograde precession in protostellar, protoplanetary, X-ray binary, non-magnetic CV DN, quasar, and black hole systems. We find that spinning, tilted CV DN systems cannot be described by a precessing ring or by a precessing rigid disk. We find that differential rotation and effects on the disk by the accretion stream must be addressed. Our analysis indicates that the best description of a retrogradely precessing spinning, tilted, CV DN accretion disk is a differentially rotating, tilted disk with an attached rotating, tilted ring located near the innermost disk annuli. In agreement with the observations and numerical simulations by others, we find that our numerically simulated CV DN accretion disks retrogradely precess as a unit. Our final, reduced expression for retrograde precession agrees well with our numerical simulation results and with selective observational systems that seem to have main-sequence secondaries. Our results suggest that a major source to retrograde precession is tidal torques like that by the Moon and the Sun on the Earth. In addition, these tidal torques should be common to a variety of systems where one member is spinning and tilted, regardless if

  7. ALIGNMENTS OF BLACK HOLES WITH THEIR WARPED ACCRETION DISKS AND EPISODIC LIFETIMES OF ACTIVE GALACTIC NUCLEI

    SciTech Connect

    Li, Yan-Rong; Wang, Jian-Min; Qiu, Jie; Cheng, Cheng

    2015-05-01

    Warped accretion disks have attracted intense attention because of their critical role in shaping the spin of supermassive massive black holes (SMBHs) through the Bardeen–Petterson effect, a general relativistic effect that leads to final alignments or anti-alignments between black holes and warped accretion disks. We study such alignment processes by explicitly taking into account the finite sizes of accretion disks and the episodic lifetimes of active galactic nuclei (AGNs) that delineate the duration of gas fueling onto accretion disks. We employ an approximate global model to simulate the evolution of accretion disks, allowing us to determine the gravitomagnetic torque that drives the alignments in a simple way. We then track down the evolutionary paths for mass and spin of black holes both in a single activity episode and over a series of episodes. Given with randomly and isotropically oriented gas fueling over episodes, we calculate the spin evolution with different episodic lifetimes and find that it is quite sensitive to the lifetimes. We therefore propose that the spin distribution of SMBHs can place constraints on the episodic lifetimes of AGNs and vice versa. The applications of our results on the observed spin distributions of SMBHs and the observed episodic lifetimes of AGNs are discussed, although both measurements at present are too ambiguous for us to draw a firm conclusion. Our prescription can be easily incorporated into semi-analytic models for black hole growth and spin evolution.

  8. Super-spinning compact objects generated by thick accretion disks

    SciTech Connect

    Li, Zilong; Bambi, Cosimo E-mail: bambi@fudan.edu.cn

    2013-03-01

    If astrophysical black hole candidates are the Kerr black holes predicted by General Relativity, the value of their spin parameter must be subject to the theoretical bound |a{sub *}| ≤ 1. In this work, we consider the possibility that these objects are either non-Kerr black holes in an alternative theory of gravity or exotic compact objects in General Relativity. We study the accretion process when their accretion disk is geometrically thick with a simple version of the Polish doughnut model. The picture of the accretion process may be qualitatively different from the one around a Kerr black hole. The inner edge of the disk may not have the typical cusp on the equatorial plane any more, but there may be two cusps, respectively above and below the equatorial plane. We extend previous work on the evolution of the spin parameter and we estimate the maximum value of a{sub *} for the super-massive black hole candidates in galactic nuclei. Since measurements of the mean radiative efficiency of AGNs require η > 0.15, we infer the ''observational'' bound |a{sub *}|∼<1.3, which seems to be quite independent of the exact nature of these objects. Such a bound is only slightly weaker than |a{sub *}|∼<1.2 found in previous work for thin disks.

  9. Maximal possible accretion rates for slim disks

    NASA Astrophysics Data System (ADS)

    Lin, Yiqing; Jiao, Chengliang

    2009-12-01

    It was proved in the previous work that there must be a maximal possible accretion rate dot M_{max} for a slim disk. Here we discuss how the value of dot M_{max} depends on the two fundamental parameters of the disk, namely the mass of the central black hole M and the viscosity parameter α. It is shown that dot M_{max} increases with decreasing α, but is almost independent of M if dot M_{max} is measured by the Eddington accretion rate dot M_{Edd} , which is in turn proportional to M.

  10. Lessons from accretion disks in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Horne, Keith

    1998-04-01

    We survey recent progress in the interpretation of observations of cataclysmic variables, whose accretion disks are heated by viscous dissipation rather than irradiation. Many features of standard viscous accretion disk models are confirmed by tomographic imaging studies of dwarf novae. Eclipse maps indicate that steady disk temperature structures are established during outbursts. Doppler maps of double-peaked emission lines suggest disk chromospheres heated by magnetic activity. Gas streams impacting on the disk rim leave expected signatures both in the eclipses and emission lines. Doppler maps of dwarf nova IP Peg at the beginning of an outburst show evidence for tidally-induced spiral shocks. While enjoying these successes, we must still face up to the dreaded ``SW Sex syndrome'' which afflicts most if not all cataclysmic variables in high accretion states. The anomalies include single-peaked emission lines with skewed kinematics, flat temperature-radius profiles, shallow offset line eclipses, and narrow low-ionization absorption lines at phase 0.5. The enigmatic behavior of AE Aqr is now largely understood in terms of a magnetic propeller model in which the rapidly spinning white dwarf magnetosphere expels the gas stream out of the system before an accretion disk can form. A final piece in this puzzle is the realization that an internal shock zone occurs in the exit stream at just the right place to explain the anomalous kinematics and violent flaring of the single-peaked emission lines. Encouraged by this success, we propose that disk-anchored magnetic propellers operate in the high accretion rate systems afflicted by the SW Sex syndrome. Magnetic fields anchored in the Keplerian disk sweep forward and apply a boost that expels gas stream material flowing above the disk plane. This working hypothesis offers a framework on which we can hang all the SW Sex anomalies. The lesson for theorists is that magnetic links appear to be transporting energy and angular

  11. Reverberation Mapping of Accretion Disk Winds in Active Galactic Nuclei

    NASA Astrophysics Data System (ADS)

    Mangham, S.

    2015-09-01

    Reverberation mapping is commonly used for determining black holes masses in AGN from the delayed response of the Broad Line Region (BLR) to fluctuations in the intensity of the AGN continuum source. However, it can also be an effective tool for investigating the structure and kinematics of the BLR itself. Much prior work has been performed to simulate the transfer functions associated with a range of basic geometries (e.g. Keplerian disks, Hubble-like outflows, etc). One promising model for the BLR is that the emission lines are formed in an equatorial accretion disk wind. Here, we predict the reverberation signatures expected from such a model, by modifying the radiative transfer and ionisation code Python that has previously been used to model broad absorption line quasars. This allows to account self-consistently for ionization and radiative transfer effects in the predicted BLR response, which are normally ignored in such calculations. We discuss the agreement between our results and prior work and consider the possibility of detecting the signature of rotating equatorial disk winds in observations obtained by velocity-resolved reverberation mapping campaigns.

  12. Poynting Jets from Accretion Disks

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Li, H.; Koldoba, A. V.; Ustyugova, G. V.; Romanova, M. M.

    2002-06-01

    We give further consideration to the problem of the evolution of a coronal, force-free magnetic field that threads a differentially rotating, conducting Keplerian disk, extending the recent work of Li and coworkers. This situation is described by the force-free Grad-Shafranov (GS) equation for the flux function Ψ(r, z) that labels the poloidal field lines (in cylindrical coordinates). The GS equation involves a function H(Ψ) describing the distribution of the poloidal current, which is determined by the differential rotation or ``twist'' of the disk that increases linearly with time. We numerically solve the GS equation in a sequence of volumes of increasing size corresponding to the expansion of the outer perfectly conducting boundaries at (Rm, Zm). The outer boundaries model the influence of an external nonmagnetized plasma. The sequence of GS solutions provides a model for the dynamical evolution of the magnetic field in response to (1) the increasing twist of the disk and (2) the pressure of external plasma. We find solutions with magnetically collimated Poynting jets in which there is a continuous outflow of energy, angular momentum, and toroidal magnetic flux from the disk into the external space. This behavior contradicts the commonly accepted ``theorem'' of solar plasma physics that the motion of the footpoints of a magnetic loop structure leads to a stationary magnetic field configuration with zero power, angular momentum, and flux outflows. In addition, we discuss magnetohydrodynamic simulations that show quasi-stationary collimated Poynting jets similar to our GS solutions. In contrast with the GS solutions, the simulations show a steady uncollimated hydromagnetic (nonforce-free) outflow from the outer part of the disk. The Poynting jets are of interest for the understanding of the jets from active galactic nuclei, microquasars, and possibly gamma-ray burst sources.

  13. Modeling Gas Distribution in Protoplanetary Accretion Disks

    NASA Astrophysics Data System (ADS)

    Kronberg, Martin; Lewis, Josiah; Brittain, Sean

    2010-07-01

    Protoplanetary accretion disks are disks of dust and gas which surround and feed material onto a forming star in the earliest stages of its evolution. One of the most useful methods for studying these disks is near infrared spectroscopy of rovibrational CO emission. This paper presents the methods in which synthetically generated spectra are modeled and fit to spectral data gathered from protoplanetary disks. This paper also discussed the methods in which this code can be improved by modifying the code to run a Monte Carlo analysis of best fit across the CONDOR cluster at Clemson University, thereby allowing for the creation of a catalog of protoplanetary disks with detailed information about them as gathered from the model.

  14. Observational constraints on black hole accretion disks

    NASA Technical Reports Server (NTRS)

    Liang, Edison P.

    1994-01-01

    We review the empirical constraints on accretion disk models of stellar-mass black holes based on recent multiwavelength observational results. In addition to time-averaged emission spectra, the time evolutions of the intensity and spectrum provide critical information about the structure, stability, and dynamics of the disk. Using the basic thermal Keplerian disk paradigm, we consider in particular generalizations of the standard optically thin disk models needed to accommodate the extremely rich variety of dynamical phenomena exhibited by black hole candidates ranging from flares of electron-positron annihilations and quasiperiodic oscillations in the X-ray intensity to X-ray novae activity. These in turn provide probes of the disk structure and global geometry. The goal is to construct a single unified framework to interpret a large variety of black hole phenomena. This paper will concentrate on the interface between basic theory and observational data modeling.

  15. Assisted Inspirals of Stellar Mass Black Holes Embedded in AGN Disks: Solving the "Final AU Problem"

    NASA Astrophysics Data System (ADS)

    Stone, Nicholas C.; Metzger, Brian D.; Haiman, Zoltán

    2016-09-01

    We explore the evolution of stellar mass black hole binaries (BHBs) which are formed in the self-gravitating disks of active galactic nuclei (AGN). Hardening due to three-body scattering and gaseous drag are effective mechanisms that reduce the semi-major axis of a BHB to radii where gravitational waves take over, on timescales shorter than the typical lifetime of the AGN disk. Taking observationally-motivated assumptions for the rate of star formation in AGN disks, we find a rate of disk-induced BHB mergers (R ˜ 3 yr^{-1} Gpc^{-3}, but with large uncertainties) that is comparable with existing estimates of the field rate of BHB mergers, and the approximate BHB merger rate implied by the recent Advanced LIGO detection of GW150914. BHBs formed thorough this channel will frequently be associated with luminous AGN, which are relatively rare within the sky error regions of future gravitational wave detector arrays. This channel could also possess a (potentially transient) electromagnetic counterpart due to super-Eddington accretion onto the stellar mass black hole following the merger.

  16. Dust Coagulation in Protoplanetary Accretion Disks

    NASA Technical Reports Server (NTRS)

    Schmitt, W.; Henning, Th.; Mucha, R.

    1996-01-01

    The time evolution of dust particles in circumstellar disk-like structures around protostars and young stellar objects is discussed. In particular, we consider the coagulation of grains due to collisional aggregation. The coagulation of the particles is calculated by solving numerically the non-linear Smoluchowski equation. The different physical processes leading to relative velocities between the grains are investigated. The relative velocities may be induced by Brownian motion, turbulence and drift motion. Starting from different regimes which can be identified during the grain growth we also discuss the evolution of dust opacities. These opacities are important for both the derivation of the circumstellar dust mass from submillimeter/millimeter continuum observations and the dynamical behavior of the disks. We present results of our numerical studies of the coagulation of dust grains in a turbulent protoplanetary accretion disk described by a time-dependent one-dimensional (radial) alpha-model. For several periods and disk radii, mass distributions of coagulated grains have been calculated. From these mass spectra, we determined the corresponding Rosseland mean dust opacities. The influence of grain opacity changes due to dust coagulation on the dynamical evolution of a protostellar disk is considered. Significant changes in the thermal structure of the protoplanetary nebula are observed. A 'gap' in the accretion disk forms at the very frontier of the coagulation, i.e., behind the sublimation boundary in the region between 1 and 5 AU.

  17. Accretion Disks and Jets Around Black Holes

    NASA Astrophysics Data System (ADS)

    Narayan, Ramesh

    2008-04-01

    Some of the most luminous objects in the universe involve accretion disks around black holes. In these systems, gas spirals into the black hole and converts a fraction of its gravitational binding energy into thermal energy and radiation. Sometimes, twin relativistic jets are ejected along the angular momentum axis of the disk. Understanding the physics of black hole accretion disks and jets is a major focus of modern astrophysics. Because the object at the center is a black hole, one must work with a relativistic theory. More importantly, one must allow for the effects of magnetic fields. These play an extremely important role, both in the extraction of angular momentum from the accreting gas -- which is what allows the gas to fall into the hole -- and in the launching, acceleration and collimation of the relativistic jets. Thus, at a minimum, one must work with the relativistic single-fluid MHD equations. The talk will briefly summarize our current understanding of black hole accretion, and outline some of the major unsolved problems.

  18. Dynamo Activity in Strongly Magnetized Accretion Disks

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Simon, Jacob B.; Armitage, Philip J.; Begelman, Mitchell C.

    2016-01-01

    Strongly magnetized accretion disks around black holes have many attractive features that may explain the enigmatic behavior observed from X-ray binaries. The physics and structure of these disks are governed by a dynamo-like mechanism, which channels the accretion power liberated by the magnetorotational instability into an ordered toroidal magnetic field. To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations. In our simulations, the strength of this self-sustained toroidal magnetic field depends on the net vertical magnetic flux we impose, which allows us to study weak-to-strong magnetization regimes. We find that the entire disk develops into a magnetic pressure-dominated state for a sufficiently strong net vertical magnetic flux. Over the two orders of magnitude in net vertical magnetic flux that we consider, the effective α-viscosity parameter scales as a power-law. We quantify dynamo properties of toroidal magnetic flux production and its buoyant escape as a function of disk magnetization. Finally, we compare our simulations to an analytic model for the vertical structure of strongly magnetized disks applicable to the high/soft state of X-ray binaries.

  19. Black hole accretion disks with coronae

    NASA Technical Reports Server (NTRS)

    Svensson, Roland; Zdziarski, Andrzej A.

    1994-01-01

    Observations suggest the existence of both hot and cold dark matter in the centers of active galactic nuclei. Recent spectral models require a major fraction of power to be dissipated in the hot matter. We study the case when the hot matter forms a corona around a standard cold alpha-disk. In particular, we investigate the case when a major fraction, f, of the power released when the cold matter accretes is transported to and dissipated in the corona. This has major effects on the cold disk, making it colder, more geometrically thin, denser, and having larger optical depths. One important consequence is the disappearance of the effectively optically thin zone as well as of the radiation pressure dominated zone for values of f sufficiently closed to unity. The disappearance of the radiation pressure dominated zone will result in a cold disk with only a gas pressure dominated zone that is stable against thermal and viscous instabilities. We also show that the pressure ( and the radiation) from the corona will only affect the surface layers of the cold disk. Our results disagree with those of other recent work on accretion disks with coronae. We find those works to be based on unphysical assumptions.

  20. Physics-Based Spectra of Accretion Disks around Black Holes

    NASA Technical Reports Server (NTRS)

    Krolik, Julian H.

    2005-01-01

    The purpose of this grant was to begin the process of deriving the light output of accretion disks around black holes directly from the actual processes that inject heat into the accreting matter, rather than from guessed dependences of heating rate on physical parameters. At JHU, the effort has focussed so far on models of accretion onto "intermediate mass black holes", a possible class of black holes, examples of which may have recently been discovered in nearby galaxies. There, Krolik and his student (Yawei Hui) have computed stellar atmospheres for uniformly-heated disks around this class of black holes. Their models serve two purposes: they are the very first serious attempts to compute the spectrum from accreting black holes in this mass range; and a library of such models can be used later in this program as contrasts for those computed on the basis of real disk dynamics. The output from these local disk calculations has also been successfully coupled to a program that applies the appropriate relativistic transformations and computes photon trajectories in order to predict the spectrum received by observers located at different polar angles. The principal new result of these calculations is the discovery of potentially observable ionization edges of H-like C and O at frequencies near the peak in flux from these objects. Most of the grant money at UCSB was spent on supporting graduate student Shane Davis. In addition. some money was spent on supporting two other students: Ari Socrates (now a Hubble Fellow at Princeton), and Laura Melling. Davis spent the year constructing stellar atmosphere models of accretion disks appropriate for the high/soft (thermal) state of black hole X-ray binaries. As with AGN models published previously by our collaboration with NASA support. our models include a complete general relativistic treatment of both the disk structure and the propagation of photons from the disk to a distant observer. They also include all important

  1. Stability of MRI-turbulent Accretion Disks

    NASA Astrophysics Data System (ADS)

    Takahashi, Hiroyuki R.; Masada, Youhei

    2011-02-01

    Based on the characteristics of the magnetorotational instability (MRI) and the MRI-driven turbulence, we construct a steady model for a geometrically thin disk using "non-standard" α-prescription. The efficiency of the angular momentum transport depends on the magnetic Prandtl number, Pm = ν/η, where ν and η are the microscopic viscous and magnetic diffusivities. In our disk model, Shakura-Sunyaev's α-parameter has a power-law dependence on the magnetic Prandtl number, that is α vprop Pm δ, where δ is the constant power-law index. Adopting Spitzer's microscopic diffusivities, the magnetic Prandtl number becomes a decreasing function of the disk radius when δ>0. The transport efficiency of the angular momentum and the viscous heating rate are thus smaller in the outer part of the disk, while these are impacted by the size of index δ. We find that the disk becomes more unstable to the gravitational instability for a larger value of index δ. The most remarkable feature of our disk model is that the thermal and secular instabilities can grow in its middle part even if the radiation pressure is negligibly small in the condition δ>2/3. In the realistic disk systems, it would be difficult to maintain the steady mass accretion state unless the Pm dependence of MRI-driven turbulence is relatively weak.

  2. Dynamics of flux tubes in accretion disks

    NASA Technical Reports Server (NTRS)

    Vishniac, E. T.; Duncan, R. C.

    1994-01-01

    The study of magnetized plasmas in astrophysics is complicated by a number of factors, not the least of which is that in considering magnetic fields in stars or accretion disks, we are considering plasmas with densities well above those we can study in the laboratory. In particular, whereas laboratory plasmas are dominated by the confining magnetic field pressure, stars, and probably accretion disks, have magnetic fields whose beta (ratio of gas pressure to magnetic field pressure) is much greater than 1. Observations of the Sun suggest that under such circumstances the magnetic field breaks apart into discrete flux tubes with a small filling factor. On the other hand, theoretical treatments of MHD turbulence in high-beta plasmas tend to assume that the field is more or less homogeneously distributed throughout the plasma. Here we consider a simple model for the distribution of magnetic flux tubes in a turbulent medium. We discuss the mechanism by which small inhomogeneities evolve into discrete flux tubes and the size and distribution of such flux tubes. We then apply the model to accretion disks. We find that the fibrilation of the magnetic field does not enhance magnetic buoyancy. We also note that the evolution of an initially diffuse field in a turbulent medium, e.g., any uniform field in a shearing flow, will initially show exponential growth as the flux tubes form. This growth saturates when the flux tube formation is complete and cannot be used as the basis for a self-sustaining dynamo effect. Since the typical state of the magnetic field is a collection of intense flux tubes, this effect is of limited interest. However, it may be important early in the evolution of the galactic magnetic field, and it will play a large role in numerical simulations. Finally, we note that the formation of flux tubes is an essential ingredient in any successful dynamo model for stars or accretion disks.

  3. Recent Observational Progress on Accretion Disks Around Compact Objects

    NASA Astrophysics Data System (ADS)

    Miller, Jon M.

    2016-04-01

    Studies of accretion disks around black holes and neutron stars over the last ten years have made remarkable progress. Our understanding of disk evolution as a function of mass accretion rate is pushing toward a consensus on thin/thick disk transitions; an apparent switching between disk-driven outflow modes has emerged; and monitoring observations have revealed complex spectral energy distributions wherein disk reprocessing must be important. Detailed studies of disk winds, in particular, have the potential to reveal the basic physical processes that mediate disk accretion, and to connect with numerical simulations. This talk will review these developments and look ahead to the potential of Astro-H.

  4. Pair production and escape in accretion disks.

    NASA Astrophysics Data System (ADS)

    Meirelles Filho, C.; Liang, E. P.

    It is shown that, in the absence of confining mechanisms, there will be a non-negligible amount of pairs escaping from the inner region of a Comptonized soft photon two-temperature accretion disk, when pair production is not balanced by annihilation. Assuming conditions such that the photons and particles in the disk can be regarded as close to a Wien plasma (Svensson, 1984), the authors calculate the rate of pair escape from the disk for both a situation close to pair balance and a situation with the rate of escape exceeding annihilation. The pairs are assumed to be created by photon-photon processes. Within this model one can account for the 511 keV γ-ray luminosity due to pair annihilation in the ISM, as recently observed in the Einstein source.

  5. Local outflows from turbulent accretion disks

    NASA Astrophysics Data System (ADS)

    Fromang, S.; Latter, H.; Lesur, G.; Ogilvie, G. I.

    2013-04-01

    Aims: The aim of this paper is to investigate the properties of accretion disks threaded by a weak vertical magnetic field, with a particular focus on the interplay between magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI) and outflows that might be launched from the disk. Methods: For that purpose, we use a set of numerical simulations performed with the MHD code RAMSES in the framework of the shearing box model. We concentrate on the case of a rather weak vertical magnetic field such that the initial ratio β0 of the thermal and magnetic pressures in the disk midplane equals 104. Results: As reported recently, we find that MHD turbulence drives an efficient outflow out of the computational box. We demonstrate a strong sensitivity of that result to the box size: enlargements in the radial and vertical directions lead to a reduction of up to an order of magnitude in the mass-loss rate. Such a dependence prevents any realistic estimates of disk mass-loss rates being derived using shearing-box simulations. We find however that the flow morphology is robust and independent of the numerical details of the simulations. Its properties display some features and approximate invariants that are reminiscent of the Blandford & Payne launching mechanism, but differences exist. For the magnetic field strength considered in this paper, we also find that angular momentum transport is most likely dominated by MHD turbulence, the saturation of which scales with the magnetic Prandtl number, the ratio of viscosity and resistivity, in a way that is in good agreement with expectations based on unstratified simulations. Conclusions: This paper thus demonstrates for the first time that accretion disks can simultaneously exhibit MRI-driven MHD turbulence along with magneto-centrifugally accelerated outflows. However, in contradiction with previously published results, such outflows probably have little impact on the disk dynamics.

  6. Black Hole Advective Accretion Disks with Optical Depth Transition

    SciTech Connect

    Artemove, Y.V.; Bisnovatyi-Kogan, G.S.; Igumenshchev, I.V.; Novikov, I.D.

    2006-02-01

    We have constructed numerically global solutions of advective accretion disks around black holes that describe a continuous transition between the effectively optically thick outer and optically thin inner disk regions. We have concentrated on models of accretion flows with large mass accretion rates, and we have employed a bridging formula for radiative losses at high and low effective optical depths.

  7. BONDI-HOYLE-LYTTLETON ACCRETION ONTO A PROTOPLANETARY DISK

    SciTech Connect

    Moeckel, Nickolas; Throop, Henry B.

    2009-12-10

    Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this 'tail-end' accretion for planet formation was recently highlighted by Throop and Bally. While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper, we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such that outer disk migrates inward, increasing the inner disk surface density and reducing the outer radius. The accretion flow also triggers the development of spiral density features, and changes to the disk inclination. The mean accretion rate onto the star remains roughly the same with and without the presence of a disk. We discuss the potential impact of this process on planet formation, including the possibility of triggered gravitational instability, inclination differences between the disk and the star, and the appearance of spiral structure in a gravitationally stable system.

  8. Variabilities of Gamma-ray Bursts from Black Hole Hyper-accretion Disks

    NASA Astrophysics Data System (ADS)

    Lin, Da-Bin; Lu, Zu-Jia; Mu, Hui-Jun; Liu, Tong; Hou, Shu-Jin; Lü, Jing; Gu, Wei-Min; Liang, En-Wei

    2016-08-01

    The emission from black hole binaries (BHBs) and active galactic nuclei (AGNs) displays significant aperiodic variabilities. The most promising explanation for these variabilities is the propagating fluctuations in the accretion flow. It is natural to expect that the mechanism driving variabilities in BHBs and AGNs may operate in a black hole hyper-accretion disk, which is believed to power gamma-ray bursts (GRBs). We study the variabilities of jet power in GRBs based on the model of propagating fluctuations. It is found that the variabilities of jet power and the temporal profile of erratic spikes in this scenario are similar to those in observed light curves of prompt gamma-ray emission of GRBs. Our results show that the mechanism driving X-ray variabilities in BHBs and AGNs may operate in the central engine to drive the variabilities of GRBs.

  9. Self-Consistent Models of Accretion Disks

    NASA Technical Reports Server (NTRS)

    Narayan, Ramesh

    1997-01-01

    The investigations of advection-dominated accretion flows (ADAFs), with emphasis on applications to X-ray binaries containing black holes and neutron stars is presented. This work is now being recognized as the standard paradigm for understanding the various spectral states of black hole X-ray Binaries (BHXBs). Topics discussed include: (1) Problem in BHXBS, namely that several of these binaries have unusually large concentrations of lithium in their companion stars; (2) A novel test to show that black holes have event horizons; (3) Application of the ADAF model to the puzzling X-ray delay in the recent outburst of the BHXB, GRO J1655-40; (4) Description of the various spectral states in BHXBS; (5) Application of the ADAF model to the famous supermassive black hole at the center of our Galaxy, Sgr A(*); (6) Writing down and solving equations describing steady-state, optically thin, advection-dominated accretion onto a Kerr black hole; (7) The effect of "photon bubble" instability on radiation dominated accretion disks; and (8) Dwarf nova disks in quiescence that have rather low magnetic Reynolds number, of order 10(exp 3).

  10. THE COSMIC BATTERY IN ASTROPHYSICAL ACCRETION DISKS

    SciTech Connect

    Contopoulos, Ioannis; Nathanail, Antonios; Katsanikas, Matthaios

    2015-06-01

    The aberrated radiation pressure at the inner edge of the accretion disk around an astrophysical black hole imparts a relative azimuthal velocity on the electrons with respect to the ions which gives rise to a ring electric current that generates large-scale poloidal magnetic field loops. This is the Cosmic Battery established by Contopoulos and Kazanas in 1998. In the present work we perform realistic numerical simulations of this important astrophysical mechanism in advection-dominated accretion flows, ADAFs. We confirm the original prediction that the inner parts of the loops are continuously advected toward the central black hole and contribute to the growth of the large-scale magnetic field, whereas the outer parts of the loops are continuously diffusing outward through the turbulent accretion flow. This process of inward advection of the axial field and outward diffusion of the return field proceeds all the way to equipartition, thus generating astrophysically significant magnetic fields on astrophysically relevant timescales. We confirm that there exists a critical value of the magnetic Prandtl number between unity and 10 in the outer disk above which the Cosmic Battery mechanism is suppressed.

  11. Hydraulic jumps in 'viscous' accretion disks

    NASA Astrophysics Data System (ADS)

    Michel, F. C.

    1984-04-01

    It is proposed that the dissipative process necessary for rapid accretion disk evolution is driven by hydraulic jump waves on the surface of the disk. These waves are excited by the asymmetric nature of the central rotator (e.g., neutron star magnetosphere) and spiral out into the disk to form a pattern corotating with the central object. Disk matter in turn is slowed slightly at each encounter with the jump and spirals inward. In this process, the disk is heated by true turbulence produced in the jumps. Additional effects, such as a systematic misalignment of the magnetic moment of the neutron star until it is nearly orthogonal, and systematic distortion of the magnetosphere in such a way as to form an even more asymmetric central 'paddle wheel', may enhance the interaction with inflowing matter. The application to X-ray sources corresponds to the 'slow' solutions of Ghosh and Lamb, and therefore to rms magnetic fields of about 4 x 10 to the 10th gauss. Analogous phenomena have been proposed to act in the formation of galactic spiral structure.

  12. The peculiar megamaser AGN NGC 1194: Comparison with the warped disk candidates NGC 1068 and NGC 4258

    NASA Astrophysics Data System (ADS)

    Fedorova , E.; Vasylenko, A.; Hnatyk, B. I.; Zhdanov, V. I.

    2016-02-01

    We analyze the X-ray properties of the Compton-thick Seyfert 1.9 radio quiet AGN in NGC 1194 using INTEGRAL (ISGRI), XMM-Newton (EPIC), Swift (BAT and XRT), and Suzaku (XIS) observations. There is a set of Fe-K lines in the NGC 1194 spectrum with complex relativistic profiles that can be considered as a sign of either a warped Bardeen-Petterson accretion disk or double black hole. We compare our results on NGC 1194 with two other megamaser warped disk candidates, NGC 1068 and NGC 4258, to trace out the other properties which can be typical for AGNs with warped accretion disks. To finally confirm or disprove the double black-hole hypotheses, further observations of the iron lines and their evolution of their shape with time are necessary. Based on obsrvations made with INTEGRAL, XMM-Newton, Swift, Suzaku.

  13. Neutrino oscillation above a black hole accretion disk

    SciTech Connect

    Malkus, A.; Kneller, J. P.; McLaughlin, G. C.; Surman, R.

    2015-05-15

    We examine neutrino oscillations in the context of an accretion disk surrounding a black hole. Because accretion disks produce large quantities of neutrinos, they may be home to interesting neutrino oscillation as well. We model accretion disks associated with stellar collapse for the sake of understanding neutrino oscillations. We find that the neutrino oscillations include phenomena seen in the protoneutron star setting as well as phenomena not seen elsewhere.

  14. Normal Modes of Black Hole Accretion Disks

    SciTech Connect

    Ortega-Rodriguez, Manuel; Silbergleit, Alexander S.; Wagoner, Robert V.; /Stanford U., Phys. Dept. /KIPAC, Menlo Park

    2006-11-07

    This paper studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing WKB techniques, we obtain the eigen frequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and examine the role of viscosity, as it appears to render some of the modes unstable to rapid growth.

  15. Accretion Disks in Massive Binary Systems

    NASA Astrophysics Data System (ADS)

    Djurašević, G. R.; Vince, I.; Atanacković, O.

    2010-12-01

    The results of our investigations of some massive close binaries (CB) (RY Sct, V448 Cyg, UU Cas and V455 Cyg), based on the photometric and spectroscopic observations indicate the existance of the accretion disk around the more massive component, located deep inside the Roche lobe. The light curve shapes of some of this systems are similar to the ones of the overcontact systems like W UMa, but the nature of these massive CBs is completely different. Here we present the models of these systems and their basic elements.

  16. Dead Zone Accretion Flows in Protostellar Disks

    NASA Technical Reports Server (NTRS)

    Turner, Neal; Sano, T.

    2008-01-01

    Planets form inside protostellar disks in a dead zone where the electrical resistivity of the gas is too high for magnetic forces to drive turbulence. We show that much of the dead zone nevertheless is active and flows toward the star while smooth, large-scale magnetic fields transfer the orbital angular momentum radially outward. Stellar X-ray and radionuclide ionization sustain a weak coupling of the dead zone gas to the magnetic fields, despite the rapid recombination of free charges on dust grains. Net radial magnetic fields are generated in the magnetorotational turbulence in the electrically conducting top and bottom surface layers of the disk, and reach the midplane by ohmic diffusion. A toroidal component to the fields is produced near the midplane by the orbital shear. The process is similar to the magnetization of the solar tachocline. The result is a laminar, magnetically driven accretion flow in the region where the planets form.

  17. Ultra-Fast Outflows in Radio-Loud AGN: New Constraints on Jet-Disk Connection

    NASA Astrophysics Data System (ADS)

    Sambruna, Rita

    There is strong observational and theoretical evidence that outflows/jets are coupled to accretion disks in black hole accreting systems, from Galactic to extragalactic sizes. While in radio-quiet AGN there is ample evidence for the presence of Ultra-Fast Outflows (UFOs) from the presence of blue-shifted absorption features in their 4-10~keV spectra, sub-relativistic winds are expected on theoretical basis in radio-loud AGN but have not been observed until now. Our recent Suzaku observations of 5 bright Broad- Line Radio Galaxies (BLRGs, the radio-loud counterparts of Seyferts) has started to change this picture. We found strong evidence for UFOs in 3 out of 5 BLRGs, with ionization parameters, column densities, and velocities of the absorber similar to Seyferts. Moreover, the outflows in BLRGs are likely to be energetically very significant: from the Suzaku data of the three sources, outflow masses similar to the accretion masses and kinetic energies of the wind similar to the X-ray luminosity and radio power of the jet are inferred. Clearly, UFOs in radio-loud AGN represent a new key ingredient to understand their central engines and in particular, the jet-disk linkage. Our discovery of UFOs in a handful of BLRGs raises the questions of how common disk winds are in radio-loud AGN, what the absorber physical and dynamical characteristics are, and what is the outflow role in broader picture of galaxy-black hole connection for radio sources, i.e., for large-scale feedback models. To address these and other issues, we propose to use archival XMM-Newton and Suzaku spectra to search for Ultra-Fast Outflows in a large number of radio sources. Over a period of two years, we will conduct a systematic, uniform analysis of the archival X-ray data, building on our extensive experience with a similar previous project for Seyferts, and using robust analysis and statistical methodologies. As an important side product, we will also obtain accurate, self- consistent measurements

  18. Accretion disk dynamics. α-viscosity in self-similar self-gravitating models

    NASA Astrophysics Data System (ADS)

    Kubsch, Marcus; Illenseer, Tobias F.; Duschl, Wolfgang J.

    2016-04-01

    Aims: We investigate the suitability of α-viscosity in self-similar models for self-gravitating disks with a focus on active galactic nuclei (AGN) disks. Methods: We use a self-similar approach to simplify the partial differential equations arising from the evolution equation, which are then solved using numerical standard procedures. Results: We find a self-similar solution for the dynamical evolution of self-gravitating α-disks and derive the significant quantities. In the Keplerian part of the disk our model is consistent with standard stationary α-disk theory, and self-consistent throughout the self-gravitating regime. Positive accretion rates throughout the disk demand a high degree of self-gravitation. Combined with the temporal decline of the accretion rate and its low amount, the model prohibits the growth of large central masses. Conclusions: α-viscosity cannot account for the evolution of the whole mass spectrum of super-massive black holes (SMBH) in AGN. However, considering the involved scales it seems suitable for modelling protoplanetary disks.

  19. Accretion disk viscosity and internal waves in disks

    NASA Astrophysics Data System (ADS)

    Huang, Min

    1992-01-01

    Recently, Vishniac, Jin and Diamond suggested that internal waves in accretion disks play a critical role in generating magnetic fields, and consequently are indirectly responsible for angular momentum transfer in thin, conducting, and non-self-gravitational disk systems. A project in which we will construct a quantitative model of the internal wave spectrum in accretion disks is started. It includes two aspects of work. The physical properties of the waves in a thin, non-self-gravitational, and non-magnetized accretion disk with realistic vertical structure is cataloged and examined. Besides the low frequency internal waves discovered by Vishniac and Diamond, it was found that sound waves with low frequency and low axisymmetry (with small absolute value of m) are capable of a driving dynamo because they are (1) well confined in a layer with thickness 2(absolute value of m)H where H is the disk scale height; (2) highly dispersive so they may survive the strong dissipation caused by the coherent nonlinear interaction their high frequency partners experience; and (3) elliptically polarized because they are confined in the z-direction. As a first step towards constructing a quantitative theory of this dynamo effect, a framework of calculating resonant nonlinear interaction among waves in disk is established. We are developing a numerical code which will compute the steady spectrum of the wave field in this framework. For simplicity, we only include the low frequency internal waves suggested by Vishniac and Diamond in the present stage. In the vicinity of the static state, the time step whose length is determined by the evolution of the modes with the largest amplitudes is too large for the modes with smaller amplitudes and overshooting occurs. Through nonlinear coupling, this overshooting is amplified and appears as a numerical instability affecting the evolution of the large amplitude modes. Shorter time steps may delay the appearance of the instability but not cure

  20. Disks and cones: interferometry of the dusty and molecular material of AGN on parsec sales

    NASA Astrophysics Data System (ADS)

    Tristam, Konrad R. W.

    2016-08-01

    The central engine of Active Galactic Nuclei (AGN) is surrounded by dense molecular and dusty material on parsec scales. Typically referred to as the ""dusty torus"", this material is a key ingredient of AGN because it (1) provides the angle dependent obscuration of the central engine and (2) most likely plays an important role for the accretion of the material onto the supermassive black hole. Observations using interferometry in the infrared have, in the last ten years, resolved and characterised the thermal emission from the dust heated by the AGN beyond simple fits of the spectral energy distribution, leading to a great leap forward in our view of the dusty material surrounding AGN. In general the torus is parsec-sized, with a large scatter in extension between individual objects. Our studies have led to the surprising discovery that the dust emission is clearly separated into two distinct components: an inner disk-like emission region which is surrounded by a polar elongated emitter. I will demonstrate these discoveries using the results obtained for the Circinus galaxy, and discuss how the results for this galaxy compare to other well studied sources. While putting strong constraints on torus models, our findings are in good qualitative agreement with recent hydrodynamic simulations of AGN tori. The next big step forward can be expected from sub-mm interferometry and I will give a short glimpse at the results from our recent ALMA observations of the outer torus in the Circinus galaxy.

  1. Constraints on Accretion Disk Physics in Low Luminosity Radio Galaxies

    NASA Astrophysics Data System (ADS)

    Baum, Stefi; Noel-Storr, Jacob; O'Dea, Christopher

    2008-03-01

    It is currently believed that essentially all galaxies harbor a massive black hole in their nuclei. If this is true, then it becomes hard to understand why we do not see the luminosity released by the inevitable accretion of the galaxy ISM onto the black hole in all galaxies. The differences in AGN output between the two classes of narrow-line radio galaxies (FRI and FRII) may hold the vital clue. High radio luminosity FRIIs generally show strong high-excitation narrow lines and are believed to be the obscured counterparts of radio loud quasars. Low radio luminosity FRIs by contrast have weaker, low-ionization lines and low ratios of optical to radio luminosities. A large difference in accretion rate and radiative efficiency between FRI and FRIIs would explain the difference in the optical properties and also provide a new unification between different classes of active galaxies in which the dominant parameter is accretion rate. Spitzer IRAC and MIPS observations already exist for most of a well defined sample of FRIs. However, the previously observed objects are the 'famous' ones, e.g., M87, M84, NGC315, 3C264, 3C31. Thus, the existing datasets are highly selected. Here we propose a very small request to complete the sample. We propose IRAC observations in all 4 bands, and MIPS photometry at 24 and 70 microns of 8, and 7 sources, respectively, for a total request of 1.7 hrs. These observations will complete the sample at very little cost in observing time. The large amount of existing complmentary data at multiple wavebands will greatly enhance the legacy value of the proposed observations. By completing the sample, the proposed IRAC and MIPS observations will produce a well defined and very well studied sample of nearby low luminosity radio galaxies. We will use the completed sample to investigate the properties of the accretion disk radiation, and the circumnuclear obscuring material.

  2. Viscosity Prescription for Gravitationally Unstable Accretion Disks

    NASA Astrophysics Data System (ADS)

    Rafikov, Roman R.

    2015-05-01

    Gravitationally unstable accretion disks emerge in a variety of astrophysical contexts—giant planet formation, FU Orioni outbursts, feeding of active galactic nuclei, and the origin of Pop III stars. When a gravitationally unstable disk is unable to cool rapidly, it settles into a quasi-stationary, fluctuating gravitoturbulent state, in which its Toomre Q remains close to a constant value {{Q}0}∼ 1. Here we develop an analytical formalism describing the evolution of such a disk, which is based on the assumptions of Q={{Q}0} and local thermal equilibrium. Our approach works in the presence of additional sources of angular momentum transport (e.g., MRI), as well as external irradiation. Thermal balance dictates a unique value of the gravitoturbulent stress {{α }gt} driving disk evolution, which is a function of the local surface density and angular frequency. We compare this approach with other commonly used gravitoturbulent viscosity prescriptions, which specify the explicit dependence of stress {{α }gt} on Toomre Q in an ad hoc fashion, and identify the ones that provide consistent results. We nevertheless argue that our Q={{Q}0} approach is more flexible, robust, and straightforward and should be given preference in applications. We illustrate this with a couple of analytical calculations—locations of the snow line and of the outer edge of the dead zone in a gravitoturbulent protoplanetary disk—which clearly show the simplicity and versatility of the Q={{Q}0} approach.

  3. Evolution and precession of accretion disk in tidal disruption events

    NASA Astrophysics Data System (ADS)

    Shen, R.-F.; Matzner, C. D.

    2012-12-01

    In a supermassive black hole (BH) tidal disruption event (TDE), the tidally disrupted star feeds the BH via an accretion disk. Most often it is assumed that the accretion rate history, hence the emission light curve, tracks the rate at which new debris mass falls back onto the disk, notably the t-5/3 power law. But this is not the case when the disk evolution due to viscous spreading - the driving force for accretion - is carefully considered. We construct a simple analytical model that comprehensively describes the accretion rate history across 4 different phases of the disk evolution, in the presence of mass fallback and disk wind loss. Accretion rate evolves differently in those phases which are governed by how the disk heat energy is carried away, early on by advection and later by radiation. The accretion rate can decline as steeply as t-5/3 only if copious disk wind loss is present during the early advection-cooled phase. Later, the accretion rate history is t-8/7 or shallower. These have great implications on the TDE flare light curve. A TDE accretion disk is most likely misaligned with the equatorial plane of the spinning BH. Moreover, in the TDE the accretion rate is super- or near-Eddington thus the disk is geometrically thick, for which case the BH's frame dragging effect may cause the disk precess as a solid body, which may manifest itself as quasi-periodic signal in the TDE light curve. Our disk evolution model predicts the disk precession period increases with time, typically as ∝ t. The results are applied to the recently jetted TDE flare Swift transient J1644 + 57 which shows numerous, quasi-periodic dips in its long-term X-ray light curve. As the current TDE sample increases, the identification of the disk precession signature provides a unique way of measuring BH spin and studying BH accretion physics.

  4. A Liquid Sodium Model of a BH Accretion Disk Dynamo

    NASA Astrophysics Data System (ADS)

    Beckley, Howard; Colgate, Stirling; Pariev, Vladimir; Weatherall, James

    2002-04-01

    A magnetic dynamo experiment is under construction at the New Mexico Institute of Mining and Technology. The experiment is designed to demonstrate in the laboratory the alpha-omega magnetic dynamo, which is believed to operate in many rotating and conducting astrophysical objects. The experiment uses the Couette flow of liquid sodium between two cylinders rotating with different angular velocities to model the omega-effect. The alpha-effect is created by the rising and expanding jets of liquid sodium driven through a pair of orifices in the end plates of the cylindrical vessel. The driven jets simulate plumes driven by buoyancy either within stars or from star-disk collisions in AGN accretion disks. A water analog of the dynamo device has been constructed and the flow necessary for the dynamo has been demonstrated. Numerical simulations of the kinematic dynamo predict that the toroidal field produced by the omega-effect will be B_phi (R_m/2pi) B_poloidal 20 x B_poloidal for the expected magnetic Reynolds number of Rm 120. The critical rate of jets necessary for the dynamo self-excitation is predicted to be a pair of jets every 4 revolutions of the outer cylinder. Within the limitations on the strength of materials and the power of the drive, the self-excitation of the dynamo appears to be feasible.

  5. WIND-ACCRETION DISKS IN WIDE BINARIES, SECOND-GENERATION PROTOPLANETARY DISKS, AND ACCRETION ONTO WHITE DWARFS

    SciTech Connect

    Perets, Hagai B.; Kenyon, Scott J.

    2013-02-20

    Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long-term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind accretion leads to long-lived stable disks over the lifetime of the asymptotic giant branch donor star. The disks have masses of a few times 10{sup -5}-10{sup -3} M {sub Sun }, with surface density and temperature profiles that follow broken power laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly, 50%-80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M {sub Sun }. When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks, suggesting that coordinated observational programs might improve our understanding of uncertain disk physics.

  6. ACCRETION IN PROTOPLANETARY DISKS BY COLLISIONAL FUSION

    SciTech Connect

    Wettlaufer, J. S.

    2010-08-10

    The formation of a solar system such as ours is believed to have followed a multi-stage process around a protostar and its associated accretion disk. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag, and Cuzzi and colleagues have shown that when midplane particle mass densities approach or exceed those of the gas, solid-solid interactions dominate the drag effect. The size dependence of the drag creates a 'bottleneck' at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Independent of whether the origin of the drag is angular momentum exchange with gas or solids in the disk, successful planetary accretion requires rapid planetesimal growth to kilometer scales. A commonly accepted picture is that for collisional velocities V{sub c} above a certain threshold value, V {sub th{approx}} 0.1-10 cm s{sup -1}, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all ranges of interparticle collision speeds the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter-phase diagrams, amorphs, and polymorphs-has been neglected. Here, it is demonstrated for compact bodies that novel aspects of surface phase transitions provide a physical basis for efficient sticking through collisional melting/amorphization/polymorphization and subsequent fusion/annealing to extend the collisional velocity range of primary accretion to {Delta}V{sub c} {approx} 1-100 m s{sup -1} >> V {sub th}, which encompasses both typical turbulent rms speeds and the velocity differences between boulder-sized and small grains {approx}1-50 m s{sup -1}. Therefore, as inspiraling meter-sized bodies collide

  7. Accretion in Protoplanetary Disks by Collisional Fusion

    NASA Astrophysics Data System (ADS)

    Wettlaufer, J. S.

    2010-08-01

    The formation of a solar system such as ours is believed to have followed a multi-stage process around a protostar and its associated accretion disk. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag, and Cuzzi and colleagues have shown that when midplane particle mass densities approach or exceed those of the gas, solid-solid interactions dominate the drag effect. The size dependence of the drag creates a "bottleneck" at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Independent of whether the origin of the drag is angular momentum exchange with gas or solids in the disk, successful planetary accretion requires rapid planetesimal growth to kilometer scales. A commonly accepted picture is that for collisional velocities Vc above a certain threshold value, V th~ 0.1-10 cm s-1, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all ranges of interparticle collision speeds the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter-phase diagrams, amorphs, and polymorphs—has been neglected. Here, it is demonstrated for compact bodies that novel aspects of surface phase transitions provide a physical basis for efficient sticking through collisional melting/amorphization/polymorphization and subsequent fusion/annealing to extend the collisional velocity range of primary accretion to ΔVc ~ 1-100 m s-1 Gt V th, which encompasses both typical turbulent rms speeds and the velocity differences between boulder-sized and small grains ~1-50 m s-1. Therefore, as inspiraling meter-sized bodies collide with smaller particles in this high velocity collisional fusion

  8. Power Spectrum Density of Stochastic Oscillating Accretion Disk

    NASA Astrophysics Data System (ADS)

    Long, G. B.; Ou, J. W.; Zheng, Y. G.

    2016-06-01

    In this paper, we employ a stochastic oscillating accretion disk model for the power spectral index and variability of BL Lac object S5 0716+714. In the model, we assume that there is a relativistic oscillation of thin accretion disks and it interacts with an external thermal bath through a friction force and a random force. We simulate the light curve and the power spectrum density (PSD) at (i) over-damped, (ii) critically damped and (iii) under-damped cases, respectively. Our results show that the simulated PSD curves depend on the intrinsic property of the accretion disk, and it could be produced in a wide interval ranging from 0.94 to 2.05 by changing the friction coefficient in a stochastic oscillating accretion disk model. We argue that accretion disk stochastic oscillating could be a possible interpretation for observed PSD variability.

  9. Optically thin accretion disks in the Kerr metric

    NASA Technical Reports Server (NTRS)

    Bjornsson, G.

    1995-01-01

    We calculate the structure of an effectively optically thin and geometrically thin accretion disk in the Kerr geometry, including electron-positron pairs. We show that the properties of the disk solutions are strongly dependent on the angular momentum of the central black hole. We find that close to a rapidly rotating hole there can be an appreciable pair density even for modest accretion rates. Pair critical accretion rates recently discovered in Newtonian disk models are also shown to be present in the general relativistic models, and we show that the geometrically thin disk approximations easily break down for rapidly rotating holes.

  10. Compact stars and accretion disks: Workshop summary

    NASA Astrophysics Data System (ADS)

    Li, J.

    1998-07-01

    A workshop on `Compact Stars and Accretion Disks' was held on 11-12 August 1997 at the Australian National University. The workshop was opened by Professor Jeremy Mould, the Director of Mount Stromlo Observatory. The workshop was organised to coincide with visits to the ANU Astrophysical Theory Centre by Professor Ron Webbink from the University of Illinois, Professor Rainer Wehrse from the University of Heidelberg and Dr Chris Tout from the University of Cambridge. The workshop attracted over 25 participants nationwide. Participants included members of the Special Research Centre for Theoretical Astrophysics, University of Sydney, led by Professor Don Melrose, Professor Dick Manchester from the ATNF, Professor Ravi Sood from ADFA, Dr John Greenhill from the University of Tasmania and Dr Rosemary Mardling from Monash University. Dr Helen Johnston from AAO and Dr Kurt Liffman from AFDL also attended the workshop. The abstracts of twelve of the workshop papers are presented in this summary.

  11. High-energy particle acceleration by explosive electromagnetic interaction in an accretion disk

    NASA Technical Reports Server (NTRS)

    Haswell, C. A.; Tajima, T.; Sakai, J.-I.

    1992-01-01

    By examining electromagnetic field evolution occurring in an accretion disk around a compact object, we arrive at an explosive mechanism of particle acceleration. Flux-freezing in the differentially rotating disk causes the seed and/or generated magnetic field to wrap up tightly, becoming highly sheared and locally predominantly azimuthal in orientation. We show how asymptotically nonlinear solutions for the electromagnetic fields may arise in isolated plasma blobs as a result of the driving of the fluid equations by the accretion flow. These fields are capable of rapidly accelerating charged particles from the disk. Acceleration through the present mechanism from AGN can give rise to energies beyond 10 exp 20 eV. Such a mechanism may present an explanation for the extragalactic origin of the most energetic observed cosmic rays.

  12. Testing Convergence for Global Accretion Disks

    NASA Astrophysics Data System (ADS)

    Hawley, John F.; Richers, Sherwood A.; Guan, Xiaoyue; Krolik, Julian H.

    2013-08-01

    Global disk simulations provide a powerful tool for investigating accretion and the underlying magnetohydrodynamic turbulence driven by magneto-rotational instability (MRI). Using them to accurately predict quantities such as stress, accretion rate, and surface brightness profile requires that purely numerical effects, arising from both resolution and algorithm, be understood and controlled. We use the flux-conservative Athena code to conduct a series of experiments on disks having a variety of magnetic topologies to determine what constitutes adequate resolution. We develop and apply several resolution metrics: langQz rang and langQ phirang, the ratio of the grid zone size to the characteristic MRI wavelength, αmag, the ratio of the Maxwell stress to the magnetic pressure, and \\langle B_R^2\\rangle /\\langle B_\\phi ^2\\rangle, the ratio of radial to toroidal magnetic field energy. For the initial conditions considered here, adequate resolution is characterized by langQz rang >= 15, langQ phirang >= 20, αmag ≈ 0.45, and \\langle B_R^2\\rangle /\\langle B_\\phi ^2\\rangle \\approx 0.2. These values are associated with >=35 zones per scaleheight H, a result consistent with shearing box simulations. Numerical algorithm is also important. Use of the Harten-Lax-van Leer-Einfeldt flux solver or second-order interpolation can significantly degrade the effective resolution compared to the Harten-Lax-van Leer discontinuities flux solver and third-order interpolation. Resolution at this standard can be achieved only with large numbers of grid zones, arranged in a fashion that matches the symmetries of the problem and the scientific goals of the simulation. Without it, however, quantitative measures important to predictions of observables are subject to large systematic errors.

  13. Prospect of the Theory of Black Hole Accretion Disks

    NASA Astrophysics Data System (ADS)

    Ju, Q.; Wang, W.

    2011-12-01

    The theory of black hole accretion disks is one of the most important basic theories and advanced topics in astrophysics. There are four known models of black hole accretion disks: standard thin disk (SSD), Shapiro-Lightman-Eardley (SLE) disk, optically thick advection dominated accretion flows (ADAF) theory (slim disk) and optically thin ADAF theory. During the past decades, they have made great contributions to the development of astrophysics. Despite their great successes in both theory and application, there are still many open questions. First of all, this paper will briefly introduce these four models. Then it will discuss several limitations of these models and review present possible solutions to address these problems. Our review work will contribute to further research of black hole accretion disks to a certain extent.

  14. The magnetic nature of disk accretion onto black holes.

    PubMed

    Miller, Jon M; Raymond, John; Fabian, Andy; Steeghs, Danny; Homan, Jeroen; Reynolds, Chris; van der Klis, Michiel; Wijnands, Rudy

    2006-06-22

    Although disk accretion onto compact objects-white dwarfs, neutron stars and black holes-is central to much of high-energy astrophysics, the mechanisms that enable this process have remained observationally difficult to determine. Accretion disks must transfer angular momentum in order for matter to travel radially inward onto the compact object. Internal viscosity from magnetic processes and disk winds can both in principle transfer angular momentum, but hitherto we lacked evidence that either occurs. Here we report that an X-ray-absorbing wind discovered in an observation of the stellar-mass black hole binary GRO J1655 - 40 (ref. 6) must be powered by a magnetic process that can also drive accretion through the disk. Detailed spectral analysis and modelling of the wind shows that it can only be powered by pressure generated by magnetic viscosity internal to the disk or magnetocentrifugal forces. This result demonstrates that disk accretion onto black holes is a fundamentally magnetic process.

  15. Magnetic fields in primordial accretion disks

    NASA Astrophysics Data System (ADS)

    Latif, M. A.; Schleicher, D. R. G.

    2016-01-01

    Magnetic fields are considered a vital ingredient of contemporary star formation and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations and are subsequently amplified by the small-scale dynamo, leading to a strong, tangled magnetic field. We explore how the magnetic field provided by the small-scale dynamo is further amplified via the α-Ω dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop. III star with 10M⊙ and an accretion rate of 10-3M⊙ yr-1, and a supermassive star with 105M⊙ and an accretion rate of 10-1M⊙ yr-1. For the 10M⊙ Pop. III star, we find that coherent magnetic fields can be produced on scales of at least 100 AU, which are sufficient to drive a jet with a luminosity of 100L⊙ and a mass outflow rate of 10-3.7M⊙ yr-1. For the supermassive star, the dynamical timescales in its environment are even shorter, implying smaller orbital timescales and an efficient magnetization out to at least 1000 AU. The jet luminosity corresponds to ~106.0L⊙ and a mass outflow rate of 10-2.1M⊙ yr-1. We expect that the feedback from the supermassive star can have a relevant impact on its host galaxy.

  16. AGN disks and black holes on the weighting scales

    NASA Astrophysics Data System (ADS)

    Huré, J.-M.; Hersant, F.; Surville, C.; Nakai, N.; Jacq, T.

    2011-06-01

    We exploit our formula for the gravitational potential of finite size, power-law disks to derive a general expression linking the mass of the black hole in active galactic nuclei (AGN), the mass of the surrounding disk, its surface density profile (through the power index s), and the differential rotation law. We find that the global rotation curve v(R) of the disk in centrifugal balance does not obey a power law of the cylindrical radius R (except in the confusing case s = -2 that mimics a Keplerian motion), and discuss the local velocity index. This formula can help to understand how, from position-velocity diagrams, mass is shared between the disk and the black hole. To this purpose, we checked the idea by generating a sample of synthetic data with different levels of Gaussian noise, added in radius. It turns out that, when observations are spread over a large radial domain and exhibit low dispersion (standard deviation σ ≲ 10% typically), the disk properties (mass and s-parameter) and black hole mass can be deduced from a non linear fit of kinematic data plotted on a (R,Rv2)-diagram. For σ ≳ 10%, masses are estimated fairly well from a linear regression (corresponding to the zeroth-order treatment of the formula), but the power index s is no longer accessible. We have applied the model to 7 AGN disks whose rotation has already been probed through water maser emission. For NGC 3393 and UGC 3789, the masses seem well constrained through the linear approach. For IC 1481, the power-law exponent s can even be deduced. Because the model is scale-free, it applies to any kind of star/disk system. Extension to disks around young stars showing deviation from Keplerian motion is thus straightforward.

  17. Accreting planets as dust dams in 'transition' disks

    SciTech Connect

    Owen, James E.

    2014-07-01

    We investigate under what circumstances an embedded planet in a protoplanetary disk may sculpt the dust distribution such that it observationally presents as a 'transition' disk. We concern ourselves with 'transition' disks that have large holes (≳ 10 AU) and high accretion rates (∼10{sup –9}-10{sup –8} M {sub ☉} yr{sup –1}), particularly, those disks which photoevaporative models struggle to explain. Adopting the observed accretion rates in 'transition' disks, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small (s ≲ 1 μm) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disk with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of a 'transition' disk (NIR dip in the spectral energy distribution (SED), millimeter cavity, and high accretion rate). At suitably high planet masses (≳ 3-4 M{sub J} ), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disk that is optically thin to infrared radiation. The planet-disk system will present as a 'transition' disk with a dip in the SED only when the planet mass and planetary accretion rate are high enough. At other times, it will present as a disk with a primordial SED, but with a cavity in the millimeter, as observed in a handful of protoplanetary disks.

  18. THE LARGE-SCALE MAGNETIC FIELDS OF THIN ACCRETION DISKS

    SciTech Connect

    Cao Xinwu; Spruit, Hendrik C. E-mail: henk@mpa-garching.mpg.de

    2013-03-10

    Large-scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large-scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared with the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number P{sub m} is around unity. In this work, we revisit this problem considering the angular momentum of the disk to be removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, {beta} {approx} 1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full three-dimensional numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong magnetororational instability turbulence surrounding them.

  19. Iron Opacity Bump Changes the Stability and Structure of Accretion Disks in Active Galactic Nuclei

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei; Davis, Shane W.; Stone, James M.

    2016-08-01

    Accretion disks around supermassive black holes have regions where the Rosseland mean opacity can be larger than the electron scattering opacity due to the large number of bound–bound transitions in iron. We study the effects of this iron opacity “bump” on the thermal stability and vertical structure of radiation-pressure-dominated accretion disks, utilizing three-dimensional radiation magnetohydrodynamic (MHD) simulations in the local shearing box approximation. The simulations self-consistently calculate the heating due to MHD turbulence caused by magneto-rotational instability and radiative cooling by using the radiative transfer module based on a variable Eddington tensor in Athena. For a 5 × 108 solar mass black hole with ˜3% of the Eddington luminosity, a model including the iron opacity bump maintains its structure for more than 10 thermal times without showing significant signs of thermal runaway. In contrast, if only electron scattering and free–free opacity are included as in the standard thin disk model, the disk collapses on the thermal timescale. The difference is caused by a combination of (1) an anti-correlation between the total optical depth and the midplane pressure, and (2) enhanced vertical advective energy transport. These results suggest that the iron opacity bump may have a strong impact on the stability and structure of active galactic nucleus (AGN) accretion disks, and may contribute to a dependence of AGN properties on metallicity. Since this opacity is relevant primarily in UV emitting regions of the flow, it may help to explain discrepancies between observation and theory that are unique to AGNs.

  20. Iron Opacity Bump Changes the Stability and Structure of Accretion Disks in Active Galactic Nuclei

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei; Davis, Shane W.; Stone, James M.

    2016-08-01

    Accretion disks around supermassive black holes have regions where the Rosseland mean opacity can be larger than the electron scattering opacity due to the large number of bound-bound transitions in iron. We study the effects of this iron opacity “bump” on the thermal stability and vertical structure of radiation-pressure-dominated accretion disks, utilizing three-dimensional radiation magnetohydrodynamic (MHD) simulations in the local shearing box approximation. The simulations self-consistently calculate the heating due to MHD turbulence caused by magneto-rotational instability and radiative cooling by using the radiative transfer module based on a variable Eddington tensor in Athena. For a 5 × 108 solar mass black hole with ˜3% of the Eddington luminosity, a model including the iron opacity bump maintains its structure for more than 10 thermal times without showing significant signs of thermal runaway. In contrast, if only electron scattering and free-free opacity are included as in the standard thin disk model, the disk collapses on the thermal timescale. The difference is caused by a combination of (1) an anti-correlation between the total optical depth and the midplane pressure, and (2) enhanced vertical advective energy transport. These results suggest that the iron opacity bump may have a strong impact on the stability and structure of active galactic nucleus (AGN) accretion disks, and may contribute to a dependence of AGN properties on metallicity. Since this opacity is relevant primarily in UV emitting regions of the flow, it may help to explain discrepancies between observation and theory that are unique to AGNs.

  1. A method for determining AGN accretion phase in field galaxies

    NASA Astrophysics Data System (ADS)

    Micic, Miroslav; Martinović, Nemanja; Sinha, Manodeep

    2016-09-01

    Recent observations of active galactic nucleus (AGN) activity in massive galaxies (log M*/ M⊙ > 10.4) show the following: (1) at z < 1, AGN-hosting galaxies do not show enhanced merger signatures compared with normal galaxies, (2) also at z < 1, most AGNs are hosted by quiescent galaxies and (3) at z > 1, the percentage of AGNs in star-forming galaxies increases and becomes comparable to the AGN percentage in quiescent galaxies at z ˜ 2. How can major mergers explain AGN activity in massive quiescent galaxies that have no merger features and no star formation to indicate a recent galaxy merger? By matching merger events in a cosmological N-body simulation to the observed AGN incidence probability in the COSMOS survey, we show that major merger-triggered AGN activity is consistent with the observations. By distinguishing between `peak' AGNs (recently merger-triggered and hosted by star-forming galaxies) and `faded' AGNs (merger-triggered a long time ago and now residing in quiescent galaxies), we show that the AGN occupation fraction in star-forming and quiescent galaxies simply follows the evolution of the galaxy merger rate. Since the galaxy merger rate drops dramatically at z < 1, the only AGNs left to be observed are the ones triggered by old mergers that are now in the declining phase of their nuclear activity, hosted by quiescent galaxies. As we go towards higher redshifts, the galaxy merger rate increases and the percentages of `peak' AGNs and `faded' AGNs become comparable.

  2. Understanding Accretion Disks through Three Dimensional Radiation MHD Simulations

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei

    I study the structures and thermal properties of black hole accretion disks in the radiation pressure dominated regime. Angular momentum transfer in the disk is provided by the turbulence generated by the magneto-rotational instability (MRI), which is calculated self-consistently with a recently developed 3D radiation magneto-hydrodynamics (MHD) code based on Athena. This code, developed by my collaborators and myself, couples both the radiation momentum and energy source terms with the ideal MHD equations by modifying the standard Godunov method to handle the stiff radiation source terms. We solve the two momentum equations of the radiation transfer equations with a variable Eddington tensor (VET), which is calculated with a time independent short characteristic module. This code is well tested and accurate in both optically thin and optically thick regimes. It is also accurate for both radiation pressure and gas pressure dominated flows. With this code, I find that when photon viscosity becomes significant, the ratio between Maxwell stress and Reynolds stress from the MRI turbulence can increase significantly with radiation pressure. The thermal instability of the radiation pressure dominated disk is then studied with vertically stratified shearing box simulations. Unlike the previous results claiming that the radiation pressure dominated disk with MRI turbulence can reach a steady state without showing any unstable behavior, I find that the radiation pressure dominated disks always either collapse or expand until we have to stop the simulations. During the thermal runaway, the heating and cooling rates from the simulations are consistent with the general criterion of thermal instability. However, details of the thermal runaway are different from the predictions of the standard alpha disk model, as many assumptions in that model are not satisfied in the simulations. We also identify the key reasons why previous simulations do not find the instability. The thermal

  3. Evolution of Pre-Main Sequence Accretion Disks

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee W.

    2003-01-01

    The aim of this project is to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, pre-main sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we are developing much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measuring disk accretion rates in these systems; and constructing detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

  4. Hydrodynamic Models of Line-Driven Accretion Disk Winds III: Local Ionization Equilibrium

    NASA Technical Reports Server (NTRS)

    Pereyra, Nicolas Antonio; Kallman, Timothy R.; White, Nicholas E. (Technical Monitor)

    2002-01-01

    We present time-dependent numerical hydrodynamic models of line-driven accretion disk winds in cataclysmic variable systems and calculate wind mass-loss rates and terminal velocities. The models are 2.5-dimensional, include an energy balance condition with radiative heating and cooling processes, and includes local ionization equilibrium introducing time dependence and spatial dependence on the line radiation force parameters. The radiation field is assumed to originate in an optically thick accretion disk. Wind ion populations are calculated under the assumption that local ionization equilibrium is determined by photoionization and radiative recombination, similar to a photoionized nebula. We find a steady wind flowing from the accretion disk. Radiative heating tends to maintain the temperature in the higher density wind regions near the disk surface, rather than cooling adiabatically. For a disk luminosity L (sub disk) = solar luminosity, white dwarf mass M(sub wd) = 0.6 solar mass, and white dwarf radii R(sub wd) = 0.01 solar radius, we obtain a wind mass-loss rate of M(sub wind) = 4 x 10(exp -12) solar mass yr(exp -1) and a terminal velocity of approximately 3000 km per second. These results confirm the general velocity and density structures found in our earlier constant ionization equilibrium adiabatic CV wind models. Further we establish here 2.5D numerical models that can be extended to QSO/AGN winds where the local ionization equilibrium will play a crucial role in the overall dynamics.

  5. Radiation Magnetohydrodynamic Simulations of the Formation of Hot Accretion Disk Coronae

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei; Stone, James M.; Davis, Shane W.

    2014-04-01

    A new mechanism to form a magnetic pressure supported, high temperature corona above the photosphere of an accretion disk is explored using three dimensional radiation magnetohydrodynamic (MHD) simulations. The thermal properties of the disk are calculated self-consistently by balancing radiative cooling through the surfaces of the disk with heating due to dissipation of turbulence driven by magneto-rotational instability (MRI). As has been noted in previous work, we find the dissipation rate per unit mass increases dramatically with height above the mid-plane, in stark contrast to the α-disk model which assumes this quantity is a constant. Thus, we find that in simulations with a low surface density (and therefore a shallow photosphere), the fraction of energy dissipated above the photosphere is significant (about 3.4% in our lowest surface density model), and this fraction increases as surface density decreases. When a significant fraction of the accretion energy is dissipated in the optically thin photosphere, the gas temperature increases substantially and a high temperature, magnetic pressure supported corona is formed. The volume-averaged temperature in the disk corona is more than 10 times larger than at the disk mid-plane. Moreover, gas temperature in the corona is strongly anti-correlated with gas density, which implies the corona formed by MRI turbulence is patchy. This mechanism to form an accretion disk corona may help explain the observed relation between the spectral index and luminosity from active galactic nucleus (AGNs), and the soft X-ray excess from some AGNs. It may also be relevant to spectral state changes in X-ray binaries.

  6. Radiation magnetohydrodynamic simulations of the formation of hot accretion disk coronae

    SciTech Connect

    Jiang, Yan-Fei; Stone, James M.; Davis, Shane W.

    2014-04-01

    A new mechanism to form a magnetic pressure supported, high temperature corona above the photosphere of an accretion disk is explored using three dimensional radiation magnetohydrodynamic (MHD) simulations. The thermal properties of the disk are calculated self-consistently by balancing radiative cooling through the surfaces of the disk with heating due to dissipation of turbulence driven by magneto-rotational instability (MRI). As has been noted in previous work, we find the dissipation rate per unit mass increases dramatically with height above the mid-plane, in stark contrast to the α-disk model which assumes this quantity is a constant. Thus, we find that in simulations with a low surface density (and therefore a shallow photosphere), the fraction of energy dissipated above the photosphere is significant (about 3.4% in our lowest surface density model), and this fraction increases as surface density decreases. When a significant fraction of the accretion energy is dissipated in the optically thin photosphere, the gas temperature increases substantially and a high temperature, magnetic pressure supported corona is formed. The volume-averaged temperature in the disk corona is more than 10 times larger than at the disk mid-plane. Moreover, gas temperature in the corona is strongly anti-correlated with gas density, which implies the corona formed by MRI turbulence is patchy. This mechanism to form an accretion disk corona may help explain the observed relation between the spectral index and luminosity from active galactic nucleus (AGNs), and the soft X-ray excess from some AGNs. It may also be relevant to spectral state changes in X-ray binaries.

  7. Stellar and quasar feedback in concert: effects on AGN accretion, obscuration, and outflows

    NASA Astrophysics Data System (ADS)

    Hopkins, Philip F.; Torrey, Paul; Faucher-Giguère, Claude-André; Quataert, Eliot; Murray, Norman

    2016-05-01

    We study the interaction of feedback from active galactic nuclei (AGN) and a multiphase interstellar medium (ISM), in simulations including explicit stellar feedback, multiphase cooling, accretion-disc winds, and Compton heating. We examine radii ˜0.1-100 pc around a black hole (BH), where the accretion rate on to the BH is determined and where AGN-powered winds and radiation couple to the ISM. We conclude: (1) the BH accretion rate is determined by exchange of angular momentum between gas and stars in gravitational instabilities. This produces accretion rates ˜0.03-1 M⊙ yr-1, sufficient to power luminous AGN. (2) The gas disc in the galactic nucleus undergoes an initial burst of star formation followed by several million years where stellar feedback suppresses the star formation rate (SFR). (3) AGN winds injected at small radii with momentum fluxes ˜LAGN/c couple efficiently to the ISM and have dramatic effects on ISM properties within ˜100 pc. AGN winds suppress the nuclear SFR by factors ˜10-30 and BH accretion rate by factors ˜3-30. They increase the outflow rate from the nucleus by factors ˜10, consistent with observational evidence for galaxy-scale AGN-driven outflows. (4) With AGN feedback, the predicted column density distribution to the BH is consistent with observations. Absent AGN feedback, the BH is isotropically obscured and there are not enough optically thin sightlines to explain type-I AGN. A `torus-like' geometry arises self-consistently as AGN feedback evacuates gas in polar regions.

  8. Continuous MHD Jet Launching from Resistive Accretion Disk

    NASA Astrophysics Data System (ADS)

    Casse, Fabien L.; Keppens, Rony

    We present numerical MHD simulations of a magnetized accretion disk launching super-fastmagnetosonic jets. These axisymmetric simulations model a time-dependant resistive accretion disk threaded by an initial vertical magnetic field. The resistivity is only important inside the disk and is prescribed as an alpha-type law where the alpha coefficient αm is smaller than unity. We show that the launching of a collimated outflow occurs self-consistently and the ejection of matter is continuous and quasi-stationary. These are the first ever 2.5D simulations of resistive accretion disks launching non-transient ideal MHD jets. This outflow is safely characterized as a jet since the flow becomes super-fastmagnetosonic well-collimated and reaches a quasi-stationary state. We present a complete illustration and explanation of the `accretion-ejection' mechanism that leads to jet formation from a magnetized accretion disk. In particular the magnetic torque inside the disk brakes the matter azimuthally and allows for accretion while it is responsible for an effective magneto-centrifugal acceleration in the jet. As such the magnetic field channels the disk angular momentum and powers the jet acceleration and collimation. The jet originates from the inner disk region where equipartition between thermal and magnetic forces is achieved.

  9. A pure hydrodynamic origin of accretion disk turbulence

    NASA Astrophysics Data System (ADS)

    Nath, Sujit Kumar; Mukhopadhyay, Banibrata

    2016-07-01

    Accretion disks consist of flows for which angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows are Rayleigh stable, but must be turbulent in order to explain observed data. Since molecular viscosity is negligible in these systems, scientists have argued for turbulent viscosity for energy dissipation and hence to explain infall of matter. However, so far, the success to explain the origin of turbulence in accretion disks is done with caveats. Here we investigate the evolution of pure hydrodynamic perturbations in stochastically driven accretion disks. We show that the accretion flows, which are inevitably driven by stochastic noise, are hydrodynamically unstable under linear perturbations. We also argue that in accretion disks, stochastic forcing appears generically due to the presence of shear between different annuli of the disk. This work resolves the turbulence problem of accretion disks from pure hydrodynamics and explains the infall of matter for both hot and cold disks. This would help in explaining the origin of timing and spectral features in the disk flows generically.

  10. Conservative GRMHD simulations of moderately thin, tilted accretion disks

    SciTech Connect

    Teixeira, Danilo Morales; Fragile, P. Chris; Zhuravlev, Viacheslav V.; Ivanov, Pavel B.

    2014-12-01

    This paper presents our latest numerical simulations of accretion disks that are misaligned with respect to the rotation axis of a Kerr black hole. In this work, we use a new, fully conservative version of the Cosmos++ general relativistic magnetohydrodynamics (GRMHD) code, coupled with an ad hoc cooling function designed to control the thickness of the disk. Together these allow us to simulate the thinnest tilted accretion disks ever using a GRMHD code. In this way, we are able to probe the regime where the dimensionless stress and scale height of the disk become comparable. We present results for both prograde and retrograde cases. The simulated prograde tilted disk shows no sign of Bardeen-Petterson alignment even in the innermost parts of the disk. The simulated retrograde tilted disk, however, does show modest alignment. The implication of these results is that the parameter space associated with Bardeen-Petterson alignment for prograde disks may be rather small, only including very thin disks. Unlike our previous work, we find no evidence for standing shocks in our simulated tilted disks. We ascribe this to the black hole spin, tilt angle, and disk scale height all being small in these simulations. We also add to the growing body of literature pointing out that the turbulence driven by the magnetorotational instability in global simulations of accretion disks is not isotropic. Finally, we provide a comparison between our moderately thin, untilted reference simulation and other numerical simulations of thin disks in the literature.

  11. LUNAR ACCRETION FROM A ROCHE-INTERIOR FLUID DISK

    SciTech Connect

    Salmon, Julien; Canup, Robin M. E-mail: robin@boulder.swri.edu

    2012-11-20

    We use a hybrid numerical approach to simulate the formation of the Moon from an impact-generated disk, consisting of a fluid model for the disk inside the Roche limit and an N-body code to describe accretion outside the Roche limit. As the inner disk spreads due to a thermally regulated viscosity, material is delivered across the Roche limit and accretes into moonlets that are added to the N-body simulation. Contrary to an accretion timescale of a few months obtained with prior pure N-body codes, here the final stage of the Moon's growth is controlled by the slow spreading of the inner disk, resulting in a total lunar accretion timescale of {approx}10{sup 2} years. It has been proposed that the inner disk may compositionally equilibrate with the Earth through diffusive mixing, which offers a potential explanation for the identical oxygen isotope compositions of the Earth and Moon. However, the mass fraction of the final Moon that is derived from the inner disk is limited by resonant torques between the disk and exterior growing moons. For initial disks containing <2.5 lunar masses (M{sub Last-Quarter-Moon }), we find that a final Moon with mass > 0.8 M{sub Last-Quarter-Moon} contains {<=}60% material derived from the inner disk, with this material preferentially delivered to the Moon at the end of its accretion.

  12. DISTRIBUTION OF ACCRETING GAS AND ANGULAR MOMENTUM ONTO CIRCUMPLANETARY DISKS

    SciTech Connect

    Tanigawa, Takayuki; Ohtsuki, Keiji; Machida, Masahiro N.

    2012-03-01

    We investigate gas accretion flow onto a circumplanetary disk from a protoplanetary disk in detail by using high-resolution three-dimensional nested-grid hydrodynamic simulations, in order to provide a basis of formation processes of satellites around giant planets. Based on detailed analyses of gas accretion flow, we find that most of gas accretion onto circumplanetary disks occurs nearly vertically toward the disk surface from high altitude, which generates a shock surface at several scale heights of the circumplanetary disk. The gas that has passed through the shock surface moves inward because its specific angular momentum is smaller than that of the local Keplerian rotation, while gas near the midplane in the protoplanetary disk cannot accrete to the circumplanetary disk. Gas near the midplane within the planet's Hill sphere spirals outward and escapes from the Hill sphere through the two Lagrangian points L{sub 1} and L{sub 2}. We also analyze fluxes of accreting mass and angular momentum in detail and find that the distributions of the fluxes onto the disk surface are well described by power-law functions and that a large fraction of gas accretion occurs at the outer region of the disk, i.e., at about 0.1 times the Hill radius. The nature of power-law functions indicates that, other than the outer edge, there is no specific radius where gas accretion is concentrated. These source functions of mass and angular momentum in the circumplanetary disk would provide us with useful constraints on the structure and evolution of the circumplanetary disk, which is important for satellite formation.

  13. Dynamo magnetic-field generation in turbulent accretion disks

    NASA Technical Reports Server (NTRS)

    Stepinski, T. F.

    1991-01-01

    Magnetic fields can play important roles in the dynamics and evolution of accretion disks. The presence of strong differential rotation and vertical density gradients in turbulent disks allows the alpha-omega dynamo mechanism to offset the turbulent dissipation and maintain strong magnetic fields. It is found that MHD dynamo magnetic-field normal modes in an accretion disk are highly localized to restricted regions of a disk. Implications for the character of real, dynamically constrained magnetic fields in accretion disks are discussed. The magnetic stress due to the mean magnetic field is found to be of the order of a viscous stress. The dominant stress, however, is likely to come from small-scale fluctuating magnetic fields. These fields may also give rise to energetic flares above the disk surface, providing a possible explanation for the highly variable hard X-ray emission from objects like Cyg X-l.

  14. FORMING AN O STAR VIA DISK ACCRETION?

    SciTech Connect

    Qiu Keping; Zhang Qizhou; Beuther, Henrik; Fallscheer, Cassandra

    2012-09-10

    We present a study of outflow, infall, and rotation in a {approx}10{sup 5} L{sub Sun} star-forming region, IRAS 18360-0537, with Submillimeter Array and IRAM 30 m observations. The 1.3 mm continuum map shows a 0.5 pc dust ridge, of which the central compact part has a mass of {approx}80 M{sub Sun} and harbors two condensations, MM1 and MM2. The CO (2-1) and SiO (5-4) maps reveal a biconical outflow centered at MM1, which is a hot molecular core (HMC) with a gas temperature of 320 {+-} 50 K and a mass of {approx}13 M{sub Sun }. The outflow has a gas mass of 54 M{sub Sun} and a dynamical timescale of 8 Multiplication-Sign 10{sup 3} yr. The kinematics of the HMC are probed by high-excitation CH{sub 3}OH and CH{sub 3}CN lines, which are detected at subarcsecond resolution and unveil a velocity gradient perpendicular to the outflow axis, suggesting a disk-like rotation of the HMC. An infalling envelope around the HMC is evidenced by CN lines exhibiting a profound inverse P Cygni profile, and the estimated mass infall rate, 1.5 Multiplication-Sign 10{sup -3} M{sub Sun} yr{sup -1}, is well comparable to that inferred from the mass outflow rate. A more detailed investigation of the kinematics of the dense gas around the HMC is obtained from the {sup 13}CO and C{sup 18}O (2-1) lines; the position-velocity diagrams of the two lines are consistent with the model of a free-falling and Keplerian-like rotating envelope. The observations suggest that the protostar of a current mass {approx}10 M{sub Sun} embedded within MM1 will develop into an O star via disk accretion and envelope infall.

  15. Forming an O Star via Disk Accretion?

    NASA Astrophysics Data System (ADS)

    Qiu, Keping; Zhang, Qizhou; Beuther, Henrik; Fallscheer, Cassandra

    2012-09-01

    We present a study of outflow, infall, and rotation in a ~105 L ⊙ star-forming region, IRAS 18360-0537, with Submillimeter Array and IRAM 30 m observations. The 1.3 mm continuum map shows a 0.5 pc dust ridge, of which the central compact part has a mass of ~80 M ⊙ and harbors two condensations, MM1 and MM2. The CO (2-1) and SiO (5-4) maps reveal a biconical outflow centered at MM1, which is a hot molecular core (HMC) with a gas temperature of 320 ± 50 K and a mass of ~13 M ⊙. The outflow has a gas mass of 54 M ⊙ and a dynamical timescale of 8 × 103 yr. The kinematics of the HMC are probed by high-excitation CH3OH and CH3CN lines, which are detected at subarcsecond resolution and unveil a velocity gradient perpendicular to the outflow axis, suggesting a disk-like rotation of the HMC. An infalling envelope around the HMC is evidenced by CN lines exhibiting a profound inverse P Cygni profile, and the estimated mass infall rate, 1.5 × 10-3 M ⊙ yr-1, is well comparable to that inferred from the mass outflow rate. A more detailed investigation of the kinematics of the dense gas around the HMC is obtained from the 13CO and C18O (2-1) lines; the position-velocity diagrams of the two lines are consistent with the model of a free-falling and Keplerian-like rotating envelope. The observations suggest that the protostar of a current mass ~10 M ⊙ embedded within MM1 will develop into an O star via disk accretion and envelope infall.

  16. Freddi: Fast Rise Exponential Decay accretion Disk model Implementation

    NASA Astrophysics Data System (ADS)

    Lipunova, G. V.; Malanchev, K. L.

    2016-10-01

    Freddi (Fast Rise Exponential Decay: accretion Disk model Implementation) solves 1-D evolution equations of the Shakura-Sunyaev accretion disk. It simulates fast rise exponential decay (FRED) light curves of low mass X-ray binaries (LMXBs). The basic equation of the viscous evolution relates the surface density and viscous stresses and is of diffusion type; evolution of the accretion rate can be found on solving the equation. The distribution of viscous stresses defines the emission from the source. The standard model for the accretion disk is implied; the inner boundary of the disk is at the ISCO or can be explicitely set. The boundary conditions in the disk are the zero stress at the inner boundary and the zero accretion rate at the outer boundary. The conditions are suitable during the outbursts in X-ray binary transients with black holes. In a binary system, the accretion disk is radially confined. In Freddi, the outer radius of the disk can be set explicitely or calculated as the position of the tidal truncation radius.

  17. TEARING UP THE DISK: HOW BLACK HOLES ACCRETE

    SciTech Connect

    Nixon, Chris; King, Andrew; Price, Daniel; Frank, Juhan

    2012-10-01

    We show that in realistic cases of accretion in active galactic nuclei or stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central regions of tilted accretion disks around spinning black holes into a set of distinct planes with only tenuous flows connecting them. If the original misalignment of the outer disk to the spin axis of the hole is 45 Degree-Sign {approx}< {theta} {approx}< 135 Degree-Sign , as in {approx}70% of randomly oriented accretion events, the continued precession of these disks sets up partially counterrotating gas flows. This drives rapid infall as angular momentum is canceled and gas attempts to circularize at smaller radii. Disk breaking close to the black hole leads to direct dynamical accretion, while breaking further out can drive gas down to scales where it can accrete rapidly. For smaller tilt angles breaking can still occur and may lead to other observable phenomena such as quasi-periodic oscillations. For such effects not to appear, the black hole spin must in practice be negligibly small, or be almost precisely aligned with the disk. Qualitatively similar results hold for any accretion disk subject to a forced differential precession, such as an external disk around a misaligned black hole binary.

  18. Accretion Disks in Supersoft X-ray Sources

    NASA Technical Reports Server (NTRS)

    Popham, Robert; DiStefano, Rosanne

    1996-01-01

    We examine the role of the accretion disk in the steady-burning white dwarf model for supersoft sources. The accretion luminosity of the disk is quite small compared to the nuclear burning luminosity of the central source. Thus, in contrast to standard accretion disks, the main role of the disk is to reprocess the radiation from the white dwarf. We calculate models of accretion disks around luminous white dwarfs and compare the resulting disk fluxes to optical and UV observations of the LMC supersoft sources CAL 83, CAL 87, and RX J0513.9-6951. We find that if the white dwarf luminosity is near the upper end of the steady-burning region, and the flaring of the disk is included, then reprocessing by the disk can account for the UV fluxes and a substantial fraction of the optical fluxes of these systems. Reprocessing by the companion star can provide additional optical flux, and here too the disk plays an important role: since the disk is fairly thick, it shadows a significant fraction of the companion's surface.

  19. MAGNETICALLY DRIVEN ACCRETION DISK WINDS AND ULTRA-FAST OUTFLOWS IN PG 1211+143

    SciTech Connect

    Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Contopoulos, Ioannis

    2015-05-20

    We present a study of X-ray ionization of MHD accretion-disk winds in an effort to constrain the physics underlying the highly ionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often detected in various sub classes of Seyfert active galactic nuclei (AGNs). Our primary focus is to show that magnetically driven outflows are indeed physically plausible candidates for the observed outflows accounting for the AGN absorption properties of the present X-ray spectroscopic observations. Employing a stratified MHD wind launched across the entire AGN accretion disk, we calculate its X-ray ionization and the ensuing X-ray absorption-line spectra. Assuming an appropriate ionizing AGN spectrum, we apply our MHD winds to model the absorption features in an XMM-Newton/EPIC spectrum of the narrow-line Seyfert, PG 1211+143. We find, through identifying the detected features with Fe Kα transitions, that the absorber has a characteristic ionization parameter of log (ξ{sub c}[erg cm s{sup −1}]) ≃ 5–6 and a column density on the order of N{sub H} ≃ 10{sup 23} cm{sup −2} outflowing at a characteristic velocity of v{sub c}/c ≃ 0.1–0.2 (where c is the speed of light). The best-fit model favors its radial location at r{sub c} ≃ 200 R{sub o} (R{sub o} is the black hole’s innermost stable circular orbit), with an inner wind truncation radius at R{sub t} ≃ 30 R{sub o}. The overall K-shell feature in the data is suggested to be dominated by Fe xxv with very little contribution from Fe xxvi and weakly ionized iron, which is in good agreement with a series of earlier analyses of the UFOs in various AGNs, including PG 1211+143.

  20. Magnetically Driven Accretion Disk Winds and Ultra-fast Outflows in PG 1211+143

    NASA Astrophysics Data System (ADS)

    Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Behar, Ehud; Contopoulos, Ioannis

    2015-05-01

    We present a study of X-ray ionization of MHD accretion-disk winds in an effort to constrain the physics underlying the highly ionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often detected in various sub classes of Seyfert active galactic nuclei (AGNs). Our primary focus is to show that magnetically driven outflows are indeed physically plausible candidates for the observed outflows accounting for the AGN absorption properties of the present X-ray spectroscopic observations. Employing a stratified MHD wind launched across the entire AGN accretion disk, we calculate its X-ray ionization and the ensuing X-ray absorption-line spectra. Assuming an appropriate ionizing AGN spectrum, we apply our MHD winds to model the absorption features in an XMM-Newton/EPIC spectrum of the narrow-line Seyfert, PG 1211+143. We find, through identifying the detected features with Fe Kα transitions, that the absorber has a characteristic ionization parameter of log (ξc[erg cm s-1]) ≃ 5-6 and a column density on the order of NH ≃ 1023 cm-2 outflowing at a characteristic velocity of vc/c ≃ 0.1-0.2 (where c is the speed of light). The best-fit model favors its radial location at rc ≃ 200 Ro (Ro is the black hole’s innermost stable circular orbit), with an inner wind truncation radius at Rt ≃ 30 Ro. The overall K-shell feature in the data is suggested to be dominated by Fe xxv with very little contribution from Fe xxvi and weakly ionized iron, which is in good agreement with a series of earlier analyses of the UFOs in various AGNs, including PG 1211+143.

  1. TLUSTY: Stellar Atmospheres, Accretion Disks, and Spectroscopic Diagnostics

    NASA Astrophysics Data System (ADS)

    Hubeny, Ivan; Lanz, Thierry

    2011-09-01

    TLUSTY is a user-oriented package written in FORTRAN77 for modeling stellar atmospheres and accretion disks and wide range of spectroscopic diagnostics. In the program's maximum configuration, the user may start from scratch and calculate a model atmosphere of a chosen degree of complexity, and end with a synthetic spectrum in a wavelength region of interest for an arbitrary stellar rotation and an arbitrary instrumental profile. The user may also model the vertical structure of annuli of an accretion disk.

  2. Simulations of accretion disks in pseudo-complex General Relativity

    NASA Astrophysics Data System (ADS)

    Hess, P. O.; Algalán B., M.; Schönenbach, T.; Greiner, W.

    2015-11-01

    After a summary on pseudo-complex General Relativity (pc-GR), circular orbits and stable orbits in general are discussed, including predictions compared to observations. Using a modified version of a model for accretions disks, presented by Page and Thorne in 1974, we apply the raytracing technique in order to simulate the appearance of an accretion disk as it should be observed in a detector. In pc-GR we predict a dark ring near a very massive, rapidly rotating object.

  3. Nonlinear dynamics of accretion disks with stochastic viscosity

    SciTech Connect

    Cowperthwaite, Philip S.; Reynolds, Christopher S.

    2014-08-20

    We present a nonlinear numerical model for a geometrically thin accretion disk with the addition of stochastic nonlinear fluctuations in the viscous parameter. These numerical realizations attempt to study the stochastic effects on the disk angular momentum transport. We show that this simple model is capable of reproducing several observed phenomenologies of accretion-driven systems. The most notable of these is the observed linear rms-flux relationship in the disk luminosity. This feature is not formally captured by the linearized disk equations used in previous work. A Fourier analysis of the dissipation and mass accretion rates across disk radii show coherence for frequencies below the local viscous frequency. This is consistent with the coherence behavior observed in astrophysical sources such as Cygnus X-1.

  4. Angular Momentum Transport in Quasi-Keplerian Accretion Disks

    NASA Astrophysics Data System (ADS)

    Subramanian, Prasad; Pujari, B. S.; Becker, Peter A.

    2004-03-01

    We reexamine arguments advanced by Hayashi & Matsuda (2001), who claim that several simple, physically motivated derivations based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsudawould radically alter our understanding of the nature of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expression for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The derivation is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.

  5. Accretion disk emission from a BL Lacertae object

    NASA Technical Reports Server (NTRS)

    Urry, C. Megan; Wandel, Amri

    1990-01-01

    The accretion disk is an attractive model for BL Lac objects because of its preferred axis and high efficiency. While the smooth continuum spectra of BL Lacs do not show large UV bumps, in marked contrast to quasars, high quality simultaneous data do reveal deviations from smoothness. Using detailed calculations of cool accretion disk spectra, the best measured ultraviolet and soft x ray spectra of the BL Lac object PKS 2155-304 are fitted. The mass and accretion rate required are determined. A hot disk or corona could comptonize soft photons from the cool disk and produce the observed power law spectrum in the 1 to 10 keV range. The dynamic time scales in the disk regions that contribute most of the observed ultraviolet and soft x ray photons are consistent with the respective time scales for intensity variations. The mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard x ray variability.

  6. High mass accretion disks: ATCA's potential for deep impact II

    NASA Astrophysics Data System (ADS)

    Walsh, Andrew; Beuther, Henrik; Longmore, Steven; Fallscheer, Cassandra

    2010-10-01

    The understanding of accretion processes and in particular of massive accretion disks is one of the most important topics in high-mass star formation. Based on our successful ATCA disk studies of high mass star formation, we now propose to investigate higher J inversion transitions of NH3 at high angular resolution (~1'') to complement our NH3 (4,4) and (5,5) data obtained last year. Last year's data showed a number of regions with clear rotational profiles, but no flattened structures that would indicate an edge-on accretion disk. We interpret our results to show rotating surrounding envelopes of any accretion disks. We were not able to see the accretion disks themselves because the (4,4) and (5,5) lines are optically thick. With observations of NH3 (7,7) and (8,8), which occur under even more extreme conditions than (4,4) or (5,5), we hope to peer through the surrounding envelope to see the accretion disks.

  7. High mass accretion disks: ATCA's potential for deep impact II

    NASA Astrophysics Data System (ADS)

    Walsh, Andrew; Beuther, Henrik; Longmore, Steven; Fallscheer, Cassandra

    2009-10-01

    The understanding of accretion processes and in particular of massive accretion disks is one of the most important topics in high-mass star formation. Based on our successful ATCA disk studies of high mass star formation, we now propose to investigate higher J inversion transitions of NH3 at high angular resolution (~1'') to complement our NH3 (4,4) and (5,5) data obtained last year. Last year's data showed a number of regions with clear rotational profiles, but no flattened structures that would indicate an edge-on accretion disk. We interpret our results to show rotating surrounding envelopes of any accretion disks. We were not able to see the accretion disks themselves because the (4,4) and (5,5) lines are optically thick. With observations of NH3 (7,7) and (8,8), which occur under even more extreme conditions than (4,4) or (5,5), we hope to peer through the surrounding envelope to see the accretion disks.

  8. THE GRAVO-MAGNETO LIMIT CYCLE IN ACCRETION DISKS

    SciTech Connect

    Martin, R. G.; Lubow, S. H.

    2011-10-10

    Previous theoretical studies have found that repeating outbursts can occur in certain regions of an accretion disk due to sudden transitions in time from gravitationally produced turbulence to magnetically produced turbulence. We analyze the disk evolution in a state diagram that plots the mass accretion rate versus disk surface density. We determine steady state accretion branches that involve gravitational and magnetic sources of turbulence. Using time-dependent numerical disk simulations, we show that cases having outbursts track along a nonsteady 'dead zone' branch and some steady state accretion branches. The outburst is the result of a rapid inter-branch transition. The gravo-magneto outbursts are then explained on this diagram as a limit cycle that is analogous to the well-known S-curve that has been applied to dwarf nova outbursts. The diagram and limit cycle provide a conceptual framework for understanding the nature of the outbursts that may occur in accretion disks of all scales, from circumplanetary to protoplanetary to active galactic nucleus accretion disks.

  9. Disk Accretion of Tidally Disrupted Rocky Bodies onto White Dwarfs

    NASA Astrophysics Data System (ADS)

    Feng, Wanda; Desch, Steven; Turner, Neal; Kalyaan, Anusha

    2016-06-01

    About 1/3 of white dwarfs (WDs) are polluted with heavy elements (e.g., Koester et al., 2014; Zuckerman et al., 2010) that should sediment out of their atmospheres on astronomically short timescales unless replenished by accretion from a reservoir, at rates that for many WDs must exceed ~1010 g/s (Farihi et al., 2010). Direct accretion of planetesimals is too improbable and Poynting-Robertson drag of dust is too slow (due to the low luminosity of WDs) (Jura, 2003), so it is often assumed that WDs accrete from a disk of gas and solid particles, fed by tidal disruption of planeteismals inside the WD Roche limit (e.g. Debes et al., 2012; Rafikov, 2011a, 2011b). A few such gaseous disks have been directly observed, through emission from Ca II atoms in the disk (e.g. Manser et al., 2016; Wilson et al. 2014). Models successfully explain the accretion rates of metals onto the WD, provided the gaseous disk viscously spreads at rates consistent with a partially suppressed magnetorotational instability (Rafikov, 2011a, 2011b). However, these models currently do not explore the likely extent of the magnetorotational instability in disks by calculating the degree of ionization, or suppression by strong magnetic field.We present a 1-D model of a gaseous WD disk accretion, to assess the extent of the magnetorotational instability in WD disks. The composition of the disk, the ionization and recombination mechanisms, and the degree of ionization of the disk are explored. Magnetic field strengths consistent with WD dipolar magnetic fields are assumed. Elsasser numbers are calculated as a function of radius in the WD disk. The rate of viscous spreading is calculated, and the model of Rafikov (2011a, 2011b) updated to compute likely accretion rates of metals onto WDs.

  10. Vertical Structure of Magnetized Accretion Disks around Young Stars

    NASA Astrophysics Data System (ADS)

    Lizano, S.; Tapia, C.; Boehler, Y.; D'Alessio, P.

    2016-01-01

    We model the vertical structure of the magnetized accretion disks that are subject to viscous and resistive heating and irradiation by the central star. We apply our formalism to the radial structure of the magnetized accretion disks that are threaded by the poloidal magnetic field dragged during the process of star formation, which was developed by Shu and coworkers. We consider disks around low-mass protostars, T Tauri, and FU Orionis stars, as well as two levels of disk magnetization: {λ }{sys}=4 (strongly magnetized disks) and {λ }{sys}=12 (weakly magnetized disks). The rotation rates of strongly magnetized disks have large deviations from Keplerian rotation. In these models, resistive heating dominates the thermal structure for the FU Ori disk, and the T Tauri disk is very thin and cold because it is strongly compressed by magnetic pressure; it may be too thin compared with observations. Instead, in the weakly magnetized disks, rotation velocities are close to Keplerian, and resistive heating is always less than 7% of the viscous heating. In these models, the T Tauri disk has a larger aspect ratio, which is consistent with that inferred from observations. All the disks have spatially extended hot atmospheres where the irradiation flux is absorbed, although most of the mass (˜90%-95%) is in the disk midplane. With the advent of ALMA one expects direct measurements of magnetic fields and their morphology at disk scales. It will then be possible to determine the mass-to-flux ratio of magnetized accretion disks around young stars, an essential parameter for their structure and evolution. Our models contribute to the understanding of the vertical structure and emission of these disks.

  11. Electromagnetic signatures of thin accretion disks in wormhole geometries

    SciTech Connect

    Harko, Tiberiu; Kovacs, Zoltan; Lobo, Francisco S. N.

    2008-10-15

    In this paper, we study the physical properties and characteristics of matter forming thin accretion disks in static and spherically symmetric wormhole spacetimes. In particular, the time averaged energy flux, the disk temperature, and the emission spectra of the accretion disks are obtained for these exotic geometries and are compared with the Schwarzschild solution. It is shown that more energy is emitted from the disk in a wormhole geometry than in the case of the Schwarzschild potential and the conversion efficiency of the accreted mass into radiation is more than a factor of 2 higher for the wormholes than for static black holes. These effects in the disk radiation are confirmed in the radial profiles of temperature corresponding to theses flux distributions, and in the emission spectrum {omega}L({omega}) of the accretion disks. We conclude that specific signatures appear in the electromagnetic spectrum, thus leading to the possibility of distinguishing wormhole geometries by using astrophysical observations of the emission spectra from accretion disks.

  12. Vertical Structure of Magnetized Accretion Disks Around Young Stars

    NASA Astrophysics Data System (ADS)

    Tapia, Carlos; Lizano, Susana

    2016-01-01

    We model the vertical structure of magnetized accretion disks subject to viscous and resistive heating, and irradiation by the central star. We apply our formalism to the radial structure of magnetized accretion disks threaded by a poloidal magnetic field dragged during the process of star formation developed by Shu and coworkers. We consider disks around low mass protostars, T Tauri, and FU Orionis stars. We consider two levels of disk magnetization, λsys = 4 (strongly magnetized disks), and λsys = 12 (weakly magnetized disks). The rotation rates of strongly magnetized disks have large deviations from Keplerian rotation. In these models, resistive heating dominates the thermal structure for the FU Ori disk. The T Tauri disk is very thin and cold because it is strongly compressed by magnetic pressure; it may be too thin compared with observations. Instead, in the weakly magnetized disks, rotation velocities are close to Keplerian, and resistive heating is always less than 7% of the viscous heating. In these models, the T Tauri disk has a larger aspect ratio, consistent with that inferred from observations. All the disks have spatially extended hot atmospheres where the irradiation flux is absorbed, although most of the mass (~ 90 - 95 %) is in the disk midplane.

  13. Accretion Disks and the Formation of Stellar Systems

    NASA Astrophysics Data System (ADS)

    Kratter, Kaitlin Michelle

    2011-02-01

    In this thesis, we examine the role of accretion disks in the formation of stellar systems, focusing on young massive disks which regulate the flow of material from the parent molecular core down to the star. We study the evolution of disks with high infall rates that develop strong gravitational instabilities. We begin in chapter 1 with a review of the observations and theory which underpin models for the earliest phases of star formation and provide a brief review of basic accretion disk physics, and the numerical methods that we employ. In chapter 2 we outline the current models of binary and multiple star formation, and review their successes and shortcomings from a theoretical and observational perspective. In chapter 3 we begin with a relatively simple analytic model for disks around young, high mass stars, showing that instability in these disks may be responsible for the higher multiplicity fraction of massive stars, and perhaps the upper mass to which they grow. We extend these models in chapter 4 to explore the properties of disks and the formation of binary companions across a broad range of stellar masses. In particular, we model the role of global and local mechanisms for angular momentum transport in regulating the relative masses of disks and stars. We follow the evolution of these disks throughout the main accretion phase of the system, and predict the trajectory of disks through parameter space. We follow up on the predictions made in our analytic models with a series of high resolution, global numerical experiments in chapter 5. Here we propose and test a new parameterization for describing rapidly accreting, gravitationally unstable disks. We find that disk properties and system multiplicity can be mapped out well in this parameter space. Finally, in chapter 6, we address whether our studies of unstable disks are relevant to recently detected massive planets on wide orbits around their central stars.

  14. Advection-Dominated Accretion Disks: Geometrically Slim or Thick?

    NASA Astrophysics Data System (ADS)

    Gu, Wei-Min; Xue, Li; Liu, Tong; Lu, Ju-Fu

    2009-12-01

    We revisit the vertical structure of black-hole accretion disks in spherical coordinates. By comparing the advective cooling with the viscous heating, we show that advection-dominated disks are geometrically thick, i.e., with a half-opening angle of Δθ > 2π/5, rather than being slim, as supposed previously in the literature.

  15. On the Structure of Accretion Disks with Outflows

    NASA Astrophysics Data System (ADS)

    Jiao, Cheng-Liang; Wu, Xue-Bing

    2011-06-01

    To study the outflows from accretion disks, we solve the set of hydrodynamic equations for accretion disks in spherical coordinates (rθphi) to obtain the explicit structure along the θ-direction. Using self-similar assumptions in the radial direction, we change the equations to a set of ordinary differential equations about the θ-coordinate, which are then solved with symmetrical boundary conditions in the equatorial plane; the velocity field is then obtained. The α viscosity prescription is applied and an advective factor f is used to simplify the energy equation. The results display thinner, quasi-Keplerian disks for Shakura-Sunyaev disks; thicker, sub-Keplerian disks for advection-dominated accretion flows; and slim disks which are consistent with previous popular analytical models. However, an inflow region and an outflow region always exist, except when the viscosity parameter α is too large, which supports the results of some recent numerical simulation works. Our results indicate that the outflows should be common in various accretion disks and may be stronger in slim disks, where both advection and radiation pressure are dominant. We also present the structure's dependence on the input parameters and discuss their physical meanings. The caveats of this work and possible improvements for the future are discussed.

  16. ON HYDROMAGNETIC STRESSES IN ACCRETION DISK BOUNDARY LAYERS

    SciTech Connect

    Pessah, Martin E.; Chan, Chi-kwan E-mail: ckch@nordita.org

    2012-05-20

    Detailed calculations of the physical structure of accretion disk boundary layers, and thus their inferred observational properties, rely on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. In order to shed light on physically viable mechanisms for angular momentum transport in this inner disk region, we examine the generation of hydromagnetic stresses and energy density in differentially rotating backgrounds with angular frequencies that increase outward in the shearing-sheet framework. We isolate the modes that are unrelated to the standard MRI and provide analytic solutions for the long-term evolution of the resulting shearing MHD waves. We show that, although the energy density of these waves can be amplified significantly, their associated stresses oscillate around zero, rendering them an inefficient mechanism to transport significant angular momentum (inward). These findings are consistent with the results obtained in numerical simulations of MHD accretion disk boundary layers and challenge the standard assumption of efficient angular momentum transport in the inner disk regions. This suggests that the detailed structure of turbulent MHD accretion disk boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity.

  17. Gravitomagnetic acceleration of accretion disk matter to polar jets

    NASA Astrophysics Data System (ADS)

    Poirier, John; Mathews, Grant

    2016-03-01

    The motion of the masses of an accretion disk around a black hole creates a general relativistic, gravitomagnetic field (GEM) from the moving matter (be it charged or uncharged) of the accretion disk. This GEM field accelerates moving masses (neutral or charged) near the accretion disk vertically upward and away from the disk, and then inward toward the axis of the disk. As the accelerated material nears the axis with approximately vertical angles, a frame dragging effect contributes to the formation of narrow jets emanating from the poles. This GEM effect is numerically evaluated in the first post Newtonian (1PN) approximation from observable quantities like the mass and velocity of the disk. This GEM force is linear in the total mass of the accretion disk matter and quadratic in the velocity of matter near to the disk with approximately the same velocity. Since these masses and velocities can be quite high in astrophysical contexts, the GEM force, which in other contexts is weak, is quite significant. This GEM effect is compared to the ordinary electromagnetic effects applied to this problem in the past.

  18. Consequences of Relativistic Neutron Outflow beyond the Accretion Disks of Active Galaxies

    NASA Astrophysics Data System (ADS)

    Ekejiuba, I. E.; Okeke, P. N.

    1993-05-01

    Three channels of relativistic electron injection in the jets of extragalactic radio sources (EGRSs) are discussed. With the assumption that an active galactic nucleus (AGN) is powered by a spinning supermassive black hole of mass ~ 10(8) M_⊙ which sits at the center of the nucleus and ingests matter and energy through an accretion disk, a model for extracting relativistic neutrons from the AGN is forged. In this model, the inelastic proton--proton and proton--photon interactions within the accretion disk, of relativistic protons with background thermal protons and photons, respectively, produce copious amounts of relativistic neutrons. These neutrons travel ballistically for ~ 10(3gamma_n ) seconds and escape from the disk before they decay. The secondary particles produced from the neutron decays then interact with the ambient magnetic field and/or other particles to produce the radio emissions observed in the jets of EGRSs. IEE acknowledges the support of the World Bank and the Federal University of Technology, Yola, Nigeria as well as the hospitality of Georgia State University.

  19. Hybrid accretion disks in active galactic nuclei. I - Structure and spectra

    NASA Technical Reports Server (NTRS)

    Wandel, Amri; Liang, Edison P.

    1991-01-01

    A unified treatment is presented of the two distinct states of vertically thin AGN accretion disks: a cool (about 10 to the 6th K) optically thick solution, and a hot (about 10 to the 9th K) optically thin solution. A generalized formalism and a new radiative cooling equation valid in both regimes are introduced. A new luminosity limit is found at which the hot and cool alpha solutions merge into a single solution of intermediate optical depth. Analytic solutions for the disk structure are given, and output spectra are computed numerically. This is used to demonstrate the prospect of fitting AGN broadband spectra containing both the UV bump as well as the hard X-ray and gamma-ray tail, using a single accretion disk model. Such models are found to make definite predictions about the observed spectrum, such as the relation between the hard X-ray spectral index, the UV-to-X-ray luminosity ratio, and a feature of about 1 MeV.

  20. Hot accretion disks with pairs: Effects of magnetic field and thermal cyclocsynchrotron radiation

    NASA Astrophysics Data System (ADS)

    Kusunose, Masaaki; Zdziarski, Andrzej A.

    1994-02-01

    We show the effects of thermal cyclosynchrotron radiation and magnetic viscosity on the structure of hot, two-temperature accretion disks. Magnetic field, B, is assumed to be randomly oriented and the ratio of magnetic pressure to either gas pressure, alpha = Pmag/Pgas, or the sum of the gas and radiation pressures, alpha = (Pmag/Pgas + Prad), is fixed. We find those effects do not change the qualitative properties of the disks, i.e., there are still two critical accretion rates related to production of e+/- pairs, (M dot)(Ucr) and (M dot)(Lcr), that affect the number of local and global disk solutions, as recently found by Bjoernsson and Svensson for the case with B = 0. However, a critical value of the alpha-viscosity parameter above which those critical accretion rates disappear becomes smaller than alphacr = 1 found in the case of B = 0, for Pmag = alpha(Pgas + Prad). If Pmag = alpha Pgas, on the other hand, alphacr is still about unity. Moreover, when Comptonized cyclosynchrotron radiation dominates Comptonized bremsstrahlung, radiation from the disk obeys a power law with the energy spectral index of approximately 0.5, in a qualitative agreement with X-ray observations of active galactic nuclei (AGNS) and Galactic black hole candidates. We also extend the hot disk solutions for Pmag = alpha(Pgas + Prad) to the effectively optically thick region, where they merge with the standard cold disk solutions. We find that the mapping method by Bjoernsson and Svensson gives a good approximation to the disk structure in the hot region and show where it breaks in the transition region. Finally, we find a region in the disk parameter space with no solutions due to the inability of Coulomb heating to supply enough energy to electrons.

  1. Observational Tests of the Picture of Disk Accretion

    NASA Astrophysics Data System (ADS)

    Maccarone, Thomas J.

    2014-09-01

    In this chapter, I present a summary of observational tests of the basic picture of disk accretion. An emphasis is placed on tests relevant to black holes, but many of the fundamental results are drawn from studies of other classes of systems. Evidence is discussed for the basic structures of accretion flows. The cases of systems with and without accretion disks are discussed, as is the evidence that disks actually form. Also discussed are the hot spots where accretion streams impact the disks, and the boundary layers in the inner parts of systems where the accretors are not black holes. The nature of slow, large amplitude variability is discussed. It is shown that some of the key predictions of the classical thermal-viscous ionization instability model for producing outbursts are in excellent agreement with observational results. It is also show that there are systems whose outbursts are extremely difficult to explain without invoking variations in the rate of mass transfer from the donor star into the outer accretion disk, or tidally induced variations in the mass transfer rates. Finally, I briefly discuss recent quasar microlensing measurements which give truly independent constraints on the inner accretion geometry around black holes.

  2. Accretion Disk Dynamics in X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Schulz, Norbert S.; Ji, Li; Nowak, M.; Canizares, C. R.; Kallman, T.

    2009-09-01

    The last decade of X-ray observations was an era of true discovery in the study of accretion phenomena in X-ray binaries. With the launch of high resolution X-ray spectrometers on board the Chandra X-ray Observatory and XMM Newton we gained novel insights in feedback processes in accretion disks. At the forefront are dynamics in winds and outflows. Recent observations now also not only reveal properties of accretion disk coronal phenomena but point us to highly variable activity in their appearance. Amongst others these include heating along the spectral branches in the Z-source Cyg X-2, short and longterm variations in the photo-ionized emissions in Cir X-1, highly variable and dynamic Ne edges in the ultra-compact binary 4U 0614+091. This presentation summarizes these recent developments and provides an outlook towards more dynamical accretion disk coronal models and perspectives for future missions.

  3. Accretion disk emission from a BL Lacertae object

    NASA Technical Reports Server (NTRS)

    Wandel, Amri; Urry, C. Megan

    1991-01-01

    It is suggested here that the UV and X-ray emission of BL Lac objects may originate in an accretion disk. Using detailed calculations of accretion disk spectra, the best-measured ultraviolet and soft X-ray spectra of the BL Lac object PKS 2155-304 are fitted, and the mass and accretion rate required is determined. The ultraviolet through soft X-ray continuum is well fitted by the spectrum of an accretion disk, but near-Eddington accretion rates are required to produce the soft X-ray excess. A hot disk or corona could Comptonize soft photons from the cool disk and produce the observed power-law spectrum in the 1-10 keV range. The dynamic time scale in the disk regions that contribute most of the observed ultraviolet and soft X-ray photons are consistent with the respective time scales for intensity variations observed in these two wave bands; the mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard X-ray variability.

  4. Implications of the β Lyrae accretion disk rim Teff

    NASA Astrophysics Data System (ADS)

    Linnell, A. P.

    2000-12-01

    Photometric evidence indicates that the massive gainer in the β Lyrae system is hidden from the observer by a thick accretion disk (Linnell, Hubeny, & Harmanec, 1998, ApJ, 509, 379). It is believed that the gainer approximates a main sequence star of Teff= 30000K. Spectroscopic analysis by Balachrandan et al. (1986, MNRAS, 219, 479) establishes a Teff of 13,300K for the donor. System synthetic spectra, fitted via the BINSYN suite to spectrophotometric scan data and IUE spectra, establish a mean rim Teff of 9000K. Assuming conservative mass transfer, Harmanec & Scholz (1993, A&A, 279, 131) use the rate of period change to derive a mass transfer rate of 20x10-6M⊙ yr-1. Connecting the rim Teff to the accretion disk face Teff with the Hubeny theory (Hubeny & Plavec 1991, AJ, 102, 1156) and using the standard accretion disk relations (Frank, King & Raine), the adopted mass transfer rate predicts a rim Teff of 4500K. The BINSYN-derived 9000K rim Teff would require a mass transfer rate 30 times larger than the adopted value. The observed rate of period change excludes such a large mass transfer rate. The bolometric luminosity of the rim, from the BINSYN model, is 5.6x1036erg sec-1. The bolometric luminosity of the gainer, on the adopted model, is 9.8x1037erg sec-1. Thus, the luminosity of the rim is 6% of the luminosity of the gainer. On the BINSYN model, the accretion disk covers 26% of the sky, as seen by the gainer. Absorption of radiation from the gainer, and its reradiation by the accretion disk, could explain the derived Teff of the rim. The conclusion is that the β Lyrae accretion disk structure must be strongly affected by radiation from the hot gainer (unseen by the observer) at the center of the accretion disk.

  5. Evolution of accretion disks in tidal disruption events

    SciTech Connect

    Shen, Rong-Feng; Matzner, Christopher D. E-mail: matzner@astro.utoronto.ca

    2014-04-01

    During a stellar tidal disruption event (TDE), an accretion disk forms as stellar debris returns to the disruption site and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. A spreading disk is a source of matter for re-accretion at rates that may exceed the later stellar fallback rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history or, at longer wavelengths, by its own emission. We model the evolution of TDE disk size and accretion rate by accounting for the time-dependent fallback rate, for the influence of wind losses in the early advective stage, and for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure-dominated states. The model provides a dynamic basis for modeling TDE light curves. All or part of a young TDE disk will precess as a solid body because of the Lense-Thirring effect, and precession may manifest itself as a quasi-periodic modulation of the light curve. The precession period increases with time. Applying our results to the jetted TDE candidate Swift J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which a main-sequence star was fully disrupted by an intermediate mass black hole on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t = 500 days corresponding to a disk transition from the advective state to the gas-pressure-dominated state.

  6. The self-regulated AGN feedback loop: the role of chaotic cold accretion

    NASA Astrophysics Data System (ADS)

    Gaspari, Massimo

    2015-08-01

    Accretion and feedback tied to supermassive black holes are known to play central role in the cosmic evolution of galaxies, groups, and clusters of galaxies. The self-regulation mechanism, that is how to link feedback and accretion, is matter of intense debate.Using high-resolution 3D hydrodynamic simulations, I discuss how the AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion. In a turbulent atmosphere heated by AGN feedback, cold clouds and filaments condense out of the hot plasma via nonlinear thermal instability, up to radii of 10s kpc, and rain toward the black hole. In the inner core, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation, boosting the accretion rate up to 100 times the Bondi rate, which is comparable to the cooling rate.Such rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. I highlight the major imprints of mechanical AGN feedback, such as buoyant bubbles, shocks, turbulence, and uplifted gas, with a critical eye toward concordance with X-ray observations. The tight self-regulation has key implications for the group/cluster scaling relations, such as Lx-Tx, in agreement with a recent X-ray stacking analysis of 250000 central galaxies.The AGN heating stifles the formation of multiphase gas, and thus accretion. Lacking the main fuel, AGN feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, chaotic cold accretion creates a symbiotic link between the black hole and the whole host galaxy, leading to a tight self-regulated feedback loop which preserves the cores of groups and clusters in quasi thermal equilibrium throughout cosmic time.

  7. The self-regulated AGN feedback loop: the role of chaotic cold accretion

    NASA Astrophysics Data System (ADS)

    Gaspari, Massimo

    2015-08-01

    Accretion and feedback tied to supermassive black holes are known to play central role in the cosmic evolution of galaxies, groups, and clusters of galaxies. The self-regulation mechanism, that is how to link feedback and accretion, is matter of intense debate.Using high-resolution 3D hydrodynamic simulations, I discuss how the AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion. In a turbulent atmosphere heated by AGN feedback, cold clouds and filaments condense out of the hot plasma via nonlinear thermal instability, up to radii of 10s kpc, and rain toward the black hole. In the inner core, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation, boosting the accretion rate up to 100 times the Bondi rate, which is comparable to the cooling rate.Such rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. I highlight the major imprints of mechanical AGN feedback, such as buoyant bubbles, shocks, turbulence, and uplifted gas, with a critical eye toward observational concordance. The tight self-regulation has key implications for the group/cluster scaling relations, such as Lx-Tx, in agreement with a recent X-ray stacking analysis of 250000 central galaxies.The AGN heating stifles the formation of multiphase gas, and thus accretion. Lacking the main fuel, AGN feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, chaotic cold accretion creates a symbiotic link between the black hole and the whole host galaxy, leading to a tight self-regulated feedback loop which preserves the cores of groups and clusters in quasi thermal equilibrium throughout cosmic time.

  8. The self-regulated AGN feedback loop: the role of chaotic cold accretion

    NASA Astrophysics Data System (ADS)

    Gaspari, Massimo

    2015-08-01

    Accretion and feedback tied to supermassive black holes are known to play central role in the cosmic evolution of galaxies, groups, and clusters of galaxies. The self-regulation mechanism, that is how to link feedback and accretion, is matter of intense debate.Using high-resolution 3D hydrodynamic simulations, I discuss how the AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion. In a turbulent atmosphere heated by AGN feedback, cold clouds and filaments condense out of the hot plasma via nonlinear thermal instability, up to radii of 10s kpc, and rain toward the black hole. In the inner core, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation, boosting the accretion rate up to 100 times the Bondi rate, which is comparable to the cooling rate.Such rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. I highlight the major imprints of mechanical AGN feedback, such as buoyant bubbles, shocks, turbulence, and uplifted gas, with a critical eye toward observational concordance. The tight self-regulation has key implications for the scaling relations, such as Lx-Tx, and the X-ray spectrum of hot halos.The AGN heating stifles the formation of multiphase gas, and thus accretion. Lacking the main fuel, AGN feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, chaotic cold accretion creates a symbiotic link between the black hole and the whole host galaxy, leading to a tight self-regulated feedback loop which preserves the cores of groups and clusters in quasi thermal equilibrium throughout cosmic time.

  9. VizieR Online Data Catalog: RM AGNs accretion rates and BH masses (Du+, 2016)

    NASA Astrophysics Data System (ADS)

    Du, P.; Wang, J.-M.; Hu, C.; Ho, L. C.; Li, Y.-R.; Bai, J.-M.

    2016-05-01

    We select all AGNs with reverberation mapping (RM) data (here only broad Hβ line), which yield robust BH mass estimates needed for our analysis. All RM AGNs before 2013 are summarized by Bentz et al. (2013ApJ...767..149B). Our project to search for super-Eddington accreting massive black holes (SEAMBHs) has monitored about 25 candidates and successfully measured Hβ lags ({tau}Hβ) in 14 AGNs to date (Du et al. 2015, J/ApJ/806/22) and other five objects monitored between 2014 and 2015 (to be submitted). See section 2 for further explanations. (2 data files).

  10. Viscous Instability Triggered by Layered Accretion in Protoplanetary Disks

    NASA Astrophysics Data System (ADS)

    Hasegawa, Yasuhiro; Takeuchi, Taku

    2015-12-01

    Layered accretion is one of the inevitable ingredients in protoplanetary disks when disk turbulence is excited by magnetorotational instabilities (MRIs). In the accretion, disk surfaces where MRIs fully operate have a high value of disk accretion rate (\\dot{M}), while the disk midplane where MRIs are generally quenched ends up with a low value of \\dot{M}. Significant progress on understanding MRIs has recently been made by a number of dedicated MHD simulations, which requires improvement of the classical treatment of α in 1D disk models. To this end, we obtain a new expression of α by utilizing an empirical formula that is derived from recent MHD simulations of stratified disks with ohmic diffusion. It is interesting that this new formulation can be regarded as a general extension of the classical α. Armed with the new α, we perform a linear stability analysis of protoplanetary disks that undergo layered accretion, and we find that a viscous instability can occur around the outer edge of dead zones. Disks become stable in using the classical α. We identify that the difference arises from Σ-dependence of \\dot{M}; whereas Σ is uniquely determined for a given value of \\dot{M} in the classical approach, the new approach leads to \\dot{M} that is a multivalued function of Σ. We confirm our finding both by exploring a parameter space and by performing the 1D, viscous evolution of disks. We finally discuss other nonideal MHD effects that are not included in our analysis but may affect our results.

  11. THERMAL EQUILIBRIA OF MAGNETICALLY SUPPORTED BLACK HOLE ACCRETION DISKS

    SciTech Connect

    Oda, H.; Machida, M.; Nakamura, K. E.; Matsumoto, R.

    2009-05-20

    We present new thermal equilibrium solutions for optically thin and optically thick disks incorporating magnetic fields. The purpose of this paper is to explain the bright hard state and the bright/slow transition observed in the rising phases of outbursts in black hole candidates. On the basis of the results of three-dimensional magnetohydrodynamic simulations, we assume that magnetic fields inside the disk are turbulent and dominated by the azimuthal component and that the azimuthally averaged Maxwell stress is proportional to the total (gas, radiation, and magnetic) pressure. We prescribe the magnetic flux advection rate to determine the azimuthal magnetic flux at a given radius. Local thermal equilibrium solutions are obtained by equating the heating, radiative cooling, and heat advection terms. We find magnetically supported ({beta} = (p {sub gas} + p {sub rad})/p {sub mag} < 1), thermally stable solutions for both optically thin disks and optically thick disks, in which the heating enhanced by the strong magnetic field balances the radiative cooling. The temperature in a low-{beta} disk (T {approx} 10{sup 7}-10{sup 11}K) is lower than that in an advection-dominated accretion flow (or radiatively inefficient accretion flow) but higher than that in a standard disk. We also study the radial dependence of the thermal equilibrium solutions. The optically thin, low-{beta} branch extends to M-dot{approx}>0.1 M-dot{sub Edd}, where M-dot is the mass accretion rate and M-dot{sub Edd} is the Eddington mass accretion rate, in which the temperature anticorrelates with the mass accretion rate. Thus, optically thin low-{beta} disks can explain the bright hard state. Optically thick, low-{beta} disks have the radial dependence of the effective temperature T {sub eff} {proportional_to} piv{sup -3/4}. Such disks will be observed as staying in a high/soft state. Furthermore, limit cycle oscillations between an optically thick low-{beta} disk and a slim disk will occur because

  12. [Predicting Spectra of Accretion Disks Around Galactic Black Holes

    NASA Technical Reports Server (NTRS)

    Krolik, Julian H.

    2004-01-01

    The purpose of this grant was to construct detailed atmosphere solutions in order to predict the spectra of accretion disks around Galactic black holes. Our plan of action was to take an existing disk atmosphere code (TLUSTY, created by Ivan Hubeny) and introduce those additional physical processes necessary to make it applicable to disks of this variety. These modifications include: treating Comptonization; introducing continuous opacity due to heavy elements; incorporating line opacity due to heavy elements; adopting a disk structure that reflects readjustments due to radiation pressure effects; and injecting heat via a physically-plausible vertical distribution.

  13. Accretion disk winds in active galactic nuclei: X-ray observations, models, and feedback

    NASA Astrophysics Data System (ADS)

    Tombesi, F.

    2016-05-01

    Powerful winds driven by active galactic nuclei (AGN) are often invoked to play a fundamental role in the evolution of both supermassive black holes (SMBHs) and their host galaxies, quenching star formation and explaining the tight SMBH-galaxy relations. A strong support of this ``quasar mode'' feedback came from the recent X-ray observation of a mildly relativistic accretion disk wind in a ultraluminous infrared galaxy (ULIRG) and its connection with a large-scale molecular outflow, providing a direct link between the SMBH and the gas out of which stars form. Spectroscopic observations, especially in the X-ray band, show that such accretion disk winds may be common in local AGN and quasars. However, their origin and characteristics are still not fully understood. Detailed theoretical models and simulations focused on radiation, magnetohydrodynamic (MHD) or a combination of these two processes to investigate the possible acceleration mechanisms and the dynamics of these winds. Some of these models have been directly compared to X-ray spectra, providing important insights into the wind physics. However, fundamental improvements on these studies will come only from the unprecedented energy resolution and sensitivity of the upcoming X-ray observatories, namely ASTRO-H (launch date early 2016) and Athena (2028).

  14. Star formation and accretion in the circumnuclear disks of active galaxies

    NASA Astrophysics Data System (ADS)

    Wutschik, Stephanie; Schleicher, Dominik R. G.; Palmer, Thomas S.

    2013-12-01

    Aims: We explore the evolution of supermassive black holes (SMBH) centered in a circumnuclear disk (CND) as a function of the mass supply from the host galaxy and considering different star formation laws, which may give rise to a self-regulation via the injection of supernova-driven turbulence. Methods: A system of equations describing star formation, black hole accretion and angular momentum transport in the disk was solved self-consistently for an axisymmetric disk in which the gravitational potential includes contributions from the black hole, the disk and the hosting galaxy. Our model extends the framework provided by Kawakatu & Wada (2008, ApJ, 681, 73), by separately considering the inner and outer part of the disk, and by introducing a potentially non-linear dependence of the star formation rate on the gas surface density and the turbulent velocity. The star formation recipes are calibrated using observational data for NGC 1097, while the accretion model is based on turbulent viscosity as a source of angular momentum transport in a thin viscous accretion disk. Results: We find that current data provide no strong constraint on the star formation recipe, and can in particular not distinguish between models entirely regulated by the surface density, and models including a dependence on the turbulent velocity. The evolution of the black hole mass, on the other hand, strongly depends on the applied star formation law, as well as the mass supply from the host galaxy. We suggest to explore the star formation process in local AGN with high-resolution ALMA observations to break the degeneracy between different star formation models.

  15. Stability of general-relativistic accretion disks

    NASA Astrophysics Data System (ADS)

    Korobkin, Oleg; Abdikamalov, Ernazar B.; Schnetter, Erik; Stergioulas, Nikolaos; Zink, Burkhard

    2011-02-01

    Self-gravitating relativistic disks around black holes can form as transient structures in a number of astrophysical scenarios such as binary neutron star and black hole-neutron star coalescences, as well as the core collapse of massive stars. We explore the stability of such disks against runaway and nonaxisymmetric instabilities using three-dimensional hydrodynamics simulations in full general relativity using the Thor code. We model the disk matter using the ideal fluid approximation with a Γ-law equation of state with Γ=4/3. We explore three disk models around nonrotating black holes with disk-to-black hole mass ratios of 0.24, 0.17, and 0.11. Because of metric blending in our initial data, all of our initial models contain an initial axisymmetric perturbation which induces radial disk oscillations. Despite these oscillations, our models do not develop the runaway instability during the first several orbital periods. Instead, all of the models develop unstable nonaxisymmetric modes on a dynamical time scale. We observe two distinct types of instabilities: the Papaloizou-Pringle and the so-called intermediate type instabilities. The development of the nonaxisymmetric mode with azimuthal number m=1 is accompanied by an outspiraling motion of the black hole, which significantly amplifies the growth rate of the m=1 mode in some cases. Overall, our simulations show that the properties of the unstable nonaxisymmetric modes in our disk models are qualitatively similar to those in the Newtonian theory.

  16. Stability of general-relativistic accretion disks

    SciTech Connect

    Korobkin, Oleg; Abdikamalov, Ernazar B.; Schnetter, Erik; Stergioulas, Nikolaos; Zink, Burkhard

    2011-02-15

    Self-gravitating relativistic disks around black holes can form as transient structures in a number of astrophysical scenarios such as binary neutron star and black hole-neutron star coalescences, as well as the core collapse of massive stars. We explore the stability of such disks against runaway and nonaxisymmetric instabilities using three-dimensional hydrodynamics simulations in full general relativity using the Thor code. We model the disk matter using the ideal fluid approximation with a {Gamma}-law equation of state with {Gamma}=4/3. We explore three disk models around nonrotating black holes with disk-to-black hole mass ratios of 0.24, 0.17, and 0.11. Because of metric blending in our initial data, all of our initial models contain an initial axisymmetric perturbation which induces radial disk oscillations. Despite these oscillations, our models do not develop the runaway instability during the first several orbital periods. Instead, all of the models develop unstable nonaxisymmetric modes on a dynamical time scale. We observe two distinct types of instabilities: the Papaloizou-Pringle and the so-called intermediate type instabilities. The development of the nonaxisymmetric mode with azimuthal number m=1 is accompanied by an outspiraling motion of the black hole, which significantly amplifies the growth rate of the m=1 mode in some cases. Overall, our simulations show that the properties of the unstable nonaxisymmetric modes in our disk models are qualitatively similar to those in the Newtonian theory.

  17. Accretion disk dynamics in X-ray binaries

    NASA Astrophysics Data System (ADS)

    Peris, Charith Srian

    Accreting X-ray binaries consist of a normal star which orbits a compact object with the former transferring matter onto the later via an accretion disk. These accretion disks emit radiation across the entire electromagnetic spectrum. This thesis exploits two regions of the spectrum, exploring the (1) inner disk regions of an accreting black hole binary, GRS1915+105, using X-ray spectral analysis and (2) the outer accretion disks of a set of neutron star and black hole binaries using Doppler Tomography applied on optical observations. X-ray spectral analysis of black hole binary GRS1915+105: GRS1915+105 stands out as an exceptional black hole primarily due to the wild variability exhibited by about half of its X-ray observations. This study focused on the steady X-ray observations of the source, which were found to exhibit significant curvature in the harder coronal component within the RXTE/PCA band-pass. The roughly constant inner-disk radius seen in a majority of the steady-soft observations is strongly reminiscent of canonical soft state black-hole binaries. Remarkably, the steady-hard observations show the presence of growing truncation in the inner-disk. A majority of the steady observations of GRS1915+105 map to the states observed in canonical black hole binaries which suggests that within the complexity of this source is a simpler underlying basis of states. Optical tomography of X-ray binary systems: Doppler tomography was applied to the strong line features present in the optical spectra of X-ray binaries in order to determine the geometric structure of the systems' emitting regions. The point where the accretion stream hits the disk, also referred to as the "hotspot'', is clearly identified in the neutron star system V691 CrA and the black hole system Nova Muscae 1991. Evidence for stream-disk overflows exist in both systems, consistent with relatively high accretion rates. In contrast, V926 Sco does not show evidence for the presence of a hotspot which

  18. Numerical simulations of accretion disks and astrophysical jets.

    NASA Astrophysics Data System (ADS)

    Rossi, P.; Bodo, G.; Mignone, A.; Massaglia, S.; Ferrari, A.

    We present the results of high resolution numerical simulations concerning accretion disks and relativistic jets. For accretion disks, in the context of the problem of angular momentum transport, we performed simulations of the magnetorotational instability in the shearing box approximation and, on the basis of our results, we discuss its validity. In the case of relativistic jets, we analyse how their interaction with the ambient medium can lead to their deceleration, as it appears to be the case for jets in FRI extragalactic radiosources.

  19. PARTICLE ACCELERATION DURING MAGNETOROTATIONAL INSTABILITY IN A COLLISIONLESS ACCRETION DISK

    SciTech Connect

    Hoshino, Masahiro

    2013-08-20

    Particle acceleration during the magnetorotational instability (MRI) in a collisionless accretion disk was investigated by using a particle-in-cell simulation. We discuss the important role that magnetic reconnection plays not only on the saturation of MRI but also on the relativistic particle generation. The plasma pressure anisotropy of p > p{sub ||} induced by the action of MRI dynamo leads to rapid growth in magnetic reconnection, resulting in the fast generation of nonthermal particles with a hard power-law spectrum. This efficient particle acceleration mechanism involved in a collisionless accretion disk may be a possible model to explain the origin of high-energy particles observed around massive black holes.

  20. ESTIMATION OF RELATIVISTIC ACCRETION DISK PARAMETERS FROM IRON LINE EMISSION

    SciTech Connect

    V. PARIEV; B. BROMLEY; W. MILLER

    2001-03-01

    The observed iron K{alpha} fluorescence lines in Seyfert I galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the emission. Here we present an analysis of the geometrical and kinematic properties of the disk based on the extreme frequency shifts of a line profile as determined by measurable flux in both the red and blue wings. The edges of the line are insensitive to the distribution of the X-ray flux over the disk, and hence provide a robust alternative to profile fitting of disk parameters. Our approach yields new, strong bounds on the inclination angle of the disk and the location of the emitting region. We apply our method to interpret observational data from MCG-6-30-15 and find that the commonly assumed inclination 30{degree} for the accretion disk in MCG-6-30-15 is inconsistent with the position of the blue edge of the line at a 3{sigma} level. A thick turbulent disk model or the presence of highly ionized iron may reconcile the bounds on inclination from the line edges with the full line profile fits based on simple, geometrically thin disk models. The bounds on the innermost radius of disk emission indicate that the black hole in MCG-6-30-15 is rotating faster than 30% of theoretical maximum. When applied to data from NGC 4151, our method gives bounds on the inclination angle of the X-ray emitting inner disk of 50 {+-} 10{degree}, consistent with the presence of an ionization cone grazing the disk as proposed by Pedlar et al. (1993). The frequency extrema analysis also provides limits to the innermost disk radius in another Seyfert 1 galaxy, NGC 3516, and is suggestive of a thick disk model.

  1. Weak Accretion in the Outer Regions of Protoplanetary Disks

    NASA Astrophysics Data System (ADS)

    Simon, Jacob B.; Bai, X.; Stone, J. M.; Armitage, P. J.; Beckwith, K.

    2013-01-01

    I will present numerical simulations of turbulence in the outer regions of protoplanetary disks. In these regions, low ionization levels and gas densities lead to weak coupling between neutral and ionized gas, enhancing the effect of ambipolar diffusion drastically. Only very thin surface layers of the disk are well ionized due to FUV photons from the central star. Our simulations focus on turbulent accretion driven by the magnetorotational instability (MRI) in the absence of a vertical magnetic field penetrating the disk. The result is a form of layered accretion, akin to the Ohmic dead zone paradigm relevant to smaller disk radii; gas is only accreted through very thin surface layers that surround a magnetically inactive "ambipolar dead zone". We find that the measured accretion rates due to this strong ambipolar diffusion are too small, by at least an order of magnitude, to account for observations. I will discuss the implications of these results for disk evolution, and a promising solution to the problem by including a vertical magnetic field.

  2. Non-LTE effects on the strength of the Lyman edge in quasar accretion disks

    NASA Technical Reports Server (NTRS)

    Stoerzer, H.; Hauschildt, P. H.; Allard, F.

    1994-01-01

    We have calculated UV/EUV (300 A which is less than or equal to lambda which is less than or equal to 1500 A) continuous energy distributions of accretion disks in the centers of active galactic nuclei (AGNs) for disk luminosities in the range 0.1 L(sub Edd) less than or equal to L(sub acc) less than 1.0 L(sub Edd) and central masses ranging from 10(exp 8) solar mass to 10(exp 9) solar mass. The vertical gas pressure structure of the disk and the disk height are obtained analytically; the temperature stratification and the resulting continuum radiation fields are calculated numerically. We have included non-Local Thermodynamic Equilibrium (LTE) effects of both the ionization equilibrium and the level populations of hydrogen and helium. We show that these non-LTE effects reduce the strength of the Lyman edge when comapred to the LTE case. In non-LTE we find that the edge can be weakly in emission or absorption for disks seen face-on, depending on the disk parameters.

  3. Massive accretion disks: ATCA's potential for deep impact

    NASA Astrophysics Data System (ADS)

    Beuther, Henrik; Longmore, Steven; Walsh, Andrew; Fallscheer, Cassandra

    2008-04-01

    The understanding of accretion processes and in particular of massive accretion disks is one of the most important topics in high-mass star formation. Based on our successful ATCA disk-pilot study of IRAS18089-1732 (Beuther & Walsh, ApJL in press), we now propose to investigate a larger sample of eleven disk candidates at high angular resolution (<1'') in the highly excited NH3(4,4)/(5,5) lines. These lines trace the densest and warmest regions and are hence well suited to isolate the accretion disks from their envelopes. The observation will reveal the kinematics of the rotating structures and allow us to differentiate whether the expected disks are in Keplerian rotation like their low-mass counterparts or not. Furthermore, the chosen line pair is well suited to investigate the temperature structure of the regions. Combining the kinematic and temperature information, we will derive detailed physical models of the rotation structures in young massive star-forming regions. Investigating a larger sample is the only way to characterize massive disks in a general way important for a comprehensive understanding of massive star formation. The ATCA with its excellent spatial resolution and sensitivity has the potential to make considerable impact in this field.

  4. On the Gravitational Stability of Gravito-turbulent Accretion Disks

    NASA Astrophysics Data System (ADS)

    Lin, Min-Kai; Kratter, Kaitlin M.

    2016-06-01

    Low mass, self-gravitating accretion disks admit quasi-steady, “gravito-turbulent” states in which cooling balances turbulent viscous heating. However, numerical simulations show that gravito-turbulence cannot be sustained beyond dynamical timescales when the cooling rate or corresponding turbulent viscosity is too large. The result is disk fragmentation. We motivate and quantify an interpretation of disk fragmentation as the inability to maintain gravito-turbulence due to formal secondary instabilities driven by: (1) cooling, which reduces pressure support; and/or (2) viscosity, which reduces rotational support. We analyze the axisymmetric gravitational stability of viscous, non-adiabatic accretion disks with internal heating, external irradiation, and cooling in the shearing box approximation. We consider parameterized cooling functions in 2D and 3D disks, as well as radiative diffusion in 3D. We show that generally there is no critical cooling rate/viscosity below which the disk is formally stable, although interesting limits appear for unstable modes with lengthscales on the order of the disk thickness. We apply this new linear theory to protoplanetary disks subject to gravito-turbulence modeled as an effective viscosity, and cooling regulated by dust opacity. We find that viscosity renders the disk beyond ˜60 au dynamically unstable on radial lengthscales a few times the local disk thickness. This is coincident with the empirical condition for disk fragmentation based on a maximum sustainable stress. We suggest turbulent stresses can play an active role in realistic disk fragmentation by removing rotational stabilization against self-gravity, and that the observed transition in behavior from gravito-turbulent to fragmenting may reflect instability of the gravito-turbulent state itself.

  5. Giant Planet Accretion in a Low-Turbulence Circumplanetary Disk

    NASA Astrophysics Data System (ADS)

    D'Angelo, Gennaro; Marzari, Francesco

    2014-06-01

    At least 5% of confirmed planets discovered by the Kepler Mission have a mass greater than Jupiter's. Gas giants more massive than Saturn account for at least 18% of all confirmed planets.The final stages of gas accretion of a giant planet occur in the presence of a circumplanetary disk (CPD). Recently, it was proposed that turbulence (and hence transport) in these disks is driven by MRI, possibly generating low-turbulence regions known as Dead Zones. It was thus suggested that gas accretion through a CPD and on the planet can be severely reduced by a Dead Zone. If CPDs create a bottleneck for the accretion of gas, then the growth of planets more massive than Jupiter may become problematic.We investigate how gas accretion on a Jupiter-mass planet is affected by a Dead Zone by means of global 3D hydrodynamics calculations. We model both the CPD and the protoplanetary disk. The accretion flow is resolved at a length scale smaller than Jupiter's radius, Rj, by using a nested-grid technique. We assume that the kinematic viscosity is constant and equal to nu=1e-5 Omega a^2, where a and Omega are respectively the planet's orbital radius and frequency. A Dead Zone around the planet is represented by a region of low viscosity (nu=1e-8 Omega a^2), extending out to ~60Rj and above and below the CPD mid-plane for a few local scale heights. We obtain an accretion rate of ~5e-5 Omega Sigma a^2, where Sigma is the unperturbed protoplanetary disk density. Calculations by D'Angelo et al. (2003) and Bate et al. (2003), which used nu=1e-5 Omega a^2 everywhere but applied a much coarser resolution and different accretion parameters, found an accretion rate of ~2e-4 Omega Sigma a^2. Accounting for variations of several tens of percent, arising from differences (between these and previous calculations) in numerical parameters and resolution, we argue that a CPD Dead Zone, as modeled here, does not significantly affect the gas accretion rate of a giant planet. This result is compatible

  6. Advection of magnetic flux by accretion disks around neutron stars

    NASA Astrophysics Data System (ADS)

    Flores-Tulian, S.; Reisenegger, A.

    The aim of our research is to address why millisecond pulsars have relatively weak surface magnetic fields, of about 10^8 G, with a narrow spread. We propose that the accretion of plasma from the companion star fully screens the original neutron star field, but the accretion disk carries additional magnetic flux from the companion star, or itself can generate field by means of dynamo processes. For a strongly magnetized star, the field prevents the disk from approaching the star. The accretion is along the field lines and deposits the matter on the polar cap. Then, the accreted plasma flows, dragging with itself the magnetic field lines, from the pole to the equator (Payne & Melatos 2004). In a following stage, when the star becomes non-magnetic, because the field has been buried, the disk touches the star. We suggest that some effective mechanism of magnetic flux transport such as that proposed by Spruit & Uzdensky 2005 (or Bisnovatyi-Kogan & Lovelace 2007), operates and necessarily leads to a "strongly magnetized disk''. It becomes laminar because the magneto-rotational instability saturates (it is considered to be responsible for turbulence in the disk), and the magnetic difussivity is negligible. Then, the loss of angular momentum allowing the accretion is only caused by the magneto-centrifugal disk-wind (Blandford & Payne 1982). Meanwhile, the wind-driven transport of the magnetic flux by the disk re-magnetizes the star. This process continues until the Lorentz force due to the star's magnetic field forbids any further accretion of matter and magnetic flux, in the Ideal Magneto-Hydro-Dynamics approach. Additional of material can fall onto the star (but at lower rate) if some instability process sets in, allowing the diffusion of mass through the magnetic field lines (e.g the Interchange Instability, Spruit & Taam 1990). All these processes might lead to an asymptotic magnetic field of 10^8 G,as is inferred from observations. We are developing a self

  7. MAGNETOROTATIONAL-INSTABILITY-DRIVEN ACCRETION IN PROTOPLANETARY DISKS

    SciTech Connect

    Bai Xuening

    2011-09-20

    Non-ideal MHD effects play an important role in the gas dynamics in protoplanetary disks (PPDs). This paper addresses the influence of non-ideal MHD effects on the magnetorotational instability (MRI) and angular momentum transport in PPDs using the most up-to-date results from numerical simulations. We perform chemistry calculations using a complex reaction network with standard prescriptions for X-ray and cosmic-ray ionizations. We first show that whether or not grains are included, the recombination time is at least one order of magnitude less than the orbital time within five disk scale heights, justifying the validity of local ionization equilibrium and strong coupling limit in PPDs. The full conductivity tensor at different disk radii and heights is evaluated, with the MRI active region determined by requiring that (1) the Ohmic Elsasser number {Lambda} be greater than 1 and (2) the ratio of gas to magnetic pressure {beta} be greater than {beta}{sub min}(Am) as identified in the recent study by Bai and Stone, where Am is the Elsasser number for ambipolar diffusion. With full flexibility as to the magnetic field strength, we provide a general framework for estimating the MRI-driven accretion rate M-dot and the magnetic field strength in the MRI active layer. We find that the MRI active layer always exists at any disk radius as long as the magnetic field in PPDs is sufficiently weak. However, the optimistically predicted M-dot in the inner disk (r = 1-10 AU) appears insufficient to account for the observed range of accretion rates in PPDs (around 10{sup -8} M{sub sun} yr{sup -1}) even in the grain-free calculation, and the presence of solar abundance sub-micron grains further reduces M-dot by one to two orders of magnitude. Moreover, we find that the predicted M-dot increases with radius in the inner disk where accretion is layered, which would lead to runaway mass accumulation if disk accretion is solely driven by the MRI. Our results suggest that stronger

  8. Structure and Spectroscopy of Black Hole Accretion Disks

    SciTech Connect

    Liedahl, D; Mauche, C

    2005-02-14

    The warped spacetime near black holes is one of the most exotic observable environments in the Universe. X-ray spectra from active galaxies obtained with the current generation of X-ray observatories reveal line emission that is modified by both special relativistic and general relativistic effects. The interpretation is that we are witnessing X-ray irradiated matter orbiting in an accretion disk around a supermassive black hole, as it prepares to cross the event horizon. This interpretation, however, is based upon highly schematized models of accretion disk structure. This report describes a project to design a detailed computer model of accretion disk atmospheres, with the goal of elucidating the high radiation density environments associated with mass flows in the curved spacetime near gravitationally collapsed objects. We have evolved the capability to generate realistic theoretical X-ray line spectra of accretion disks, thereby providing the means for a workable exploration of the behavior of matter in the strong-field limit of gravitation.

  9. Durability of the accretion disk of millisecond pulsars.

    PubMed

    Michel, F C; Dessler, A J

    1985-05-24

    Pulsars with pulsation periods in the millisecond range are thought to be neutron stars that have acquired an extraordinarily short spin period through the accretion of stellar material spiraling down onto the neutron star from a nearby companion. Nearly all the angular momentum and most of the mass of the companion star is transferred to the neutron star. During this process, wherein the neutron star consumes its companion, it is required that a disk of stellar material be formed around the neutron star. In conventional models it is supposed that the disk is somehow lost when the accretion phase is finished, so that only the rapidly spinning neutron star remains. However, it is possible that, after the accretion phase, a residual disk remains in stable orbit around the neutron star. The end result of such an accretion process is an object that looks much like a miniature (about 100 kilometers), heavy version of Saturn: a central object (the neutron star) surrounded by a durable disk. PMID:17797665

  10. Accretion outbursts in self-gravitating protoplanetary disks

    SciTech Connect

    Bae, Jaehan; Hartmann, Lee; Zhu, Zhaohuan; Nelson, Richard P. E-mail: lhartm@umich.edu E-mail: r.p.nelson@qmul.ac.uk

    2014-11-01

    We improve on our previous treatments of the long-term evolution of protostellar disks by explicitly solving disk self-gravity in two dimensions. The current model is an extension of the one-dimensional layered accretion disk model of Bae et al. We find that gravitational instability (GI)-induced spiral density waves heat disks via compressional heating (i.e., PdV work), and can trigger accretion outbursts by activating the magnetorotational instability (MRI) in the magnetically inert disk dead zone. The GI-induced spiral waves propagate well inside of the gravitationally unstable region before they trigger outbursts at R ≲ 1 AU where GI cannot be sustained. This long-range propagation of waves cannot be reproduced with the previously used local α treatments for GI. In our standard model where zero dead-zone residual viscosity (α{sub rd}) is assumed, the GI-induced stress measured at the onset of outbursts is locally as large as 0.01 in terms of the generic α parameter. However, as suggested in our previous one-dimensional calculations, we confirm that the presence of a small but finite α{sub rd} triggers thermally driven bursts of accretion instead of the GI + MRI-driven outbursts that are observed when α{sub rd} = 0. The inclusion of non-zero residual viscosity in the dead zone decreases the importance of GI soon after mass feeding from the envelope cloud ceases. During the infall phase while the central protostar is still embedded, our models stay in a 'quiescent' accretion phase with M-dot {sub acc}∼10{sup −8}--10{sup −7} M{sub ⊙} yr{sup −1} over 60% of the time and spend less than 15% of the infall phase in accretion outbursts. While our models indicate that episodic mass accretion during protostellar evolution can qualitatively help explain the low accretion luminosities seen in most low-mass protostars, detailed tests of the mechanism will require model calculations for a range of protostellar masses with some constraint on the initial core

  11. Tilted Thick-Disk Accretion onto a Kerr Black Hole

    SciTech Connect

    Fragile, P C; Anninos, P

    2003-12-12

    We present the first results from fully general relativistic numerical studies of thick-disk accretion onto a rapidly-rotating (Kerr) black hole with a spin axis that is tilted (not aligned) with the angular momentum vector of the disk. We initialize the problem with the solution for an aligned, constant angular momentum, accreting thick disk around a black hole with spin a/M = J/M{sup 2} = +0.9 (prograde disk). The black hole is then instantaneously tilted, through a change in the metric, by an angle {beta}{sub 0}. In this Letter we report results with {beta}{sub 0} = 0, 15, and 30{sup o}. The disk is allowed to respond to the Lense-Thirring precession of the tilted black hole. We find that the disk settles into a quasi-static, twisted, warped configuration with Lense-Thirring precession dominating out to a radius analogous to the Bardeen-Petterson transition in tilted Keplerian disks.

  12. Ultraviolet line diagnostics of accretion disk winds in cataclysmic variables

    NASA Technical Reports Server (NTRS)

    Vitello, Peter; Shlosman, Isaac

    1993-01-01

    The IUE data base is used to analyze the UV line shapes of the cataclysmic variables RW Sex, RW Tri, and V Sge. Observed lines are compared to synthetic line profiles computed using a model of rotating biconical winds from accretion disks. The wind model calculates the wind ionization structure self-consistently including photoionization from the disk and boundary layer and treats 3D line radiation transfer in the Sobolev approximation. It is found that winds from accretion disks provide a good fit for reasonable parameters to the observed UV lines which include the P Cygni profiles for low-inclination systems and pure emission at large inclination. Disk winds are preferable to spherical winds which originate on the white dwarf because they: (1) require a much lower ratio of mass-loss rate to accretion rate and are therefore more plausible energetically; (2) provide a natural source for a biconical distribution of mass outflow which produces strong scattering far above the disk leading to P Cygni profiles for low-inclination systems and pure line emission profiles at high inclination with the absence of eclipses in UV lines; and (3) produce rotation-broadened pure emission lines at high inclination.

  13. UV line diagnostics of accretion disk winds in cataclysmic variables

    NASA Technical Reports Server (NTRS)

    Vitello, Peter; Shlosman, Isaac

    1992-01-01

    The IUE data base is used to analyze the UV line shapes of cataclysmic variables RW Sex, RW Tri, and V Sge. Observed lines are compared to synthetic line profiles computed using a model of rotating bi-conical winds from accretion disks. The wind model calculates the wind ionization structure self-consistently including photoionization from the disk and boundary layer and treats 3-D line radiation transfer in the Sobolev approximation. It is found that winds from accretion disks provide a good fit for reasonable parameters to the observed UV lines which include the P Cygni profiles for low inclination systems and pure emission at large inclination. Disk winds are preferable to spherical winds which originate on the white dwarf because they (1) require a much lower ratio of mass loss rate to accretion rate and are therefore more plausible energetically, (2) provide a natural source for a bi-conical distribution of mass outflow which produces strong scattering far above the disk leading to P Cygni profiles for low inclination systems, and pure line emission profiles at high inclination with the absence of eclipses in UV lines, and (3) produce rotation broadened pure emission lines at high inclination.

  14. Jets and Accretion Disks in X-ray Binaries

    NASA Astrophysics Data System (ADS)

    Tomsick, John

    The outflow of material in the form of jets is a common phenomenon in astronomical sources with accretion disks. Even though jets are seen coming from the cores of galaxies, Galactic compact objects in X-ray binaries, and stars as they are forming, we do not understand in detail what accretion disk conditions are necessary to support a relativistic jet. This proposal focuses on multi-wavelength studies of X-ray binaries in order to improve our understanding of the connection between the disk and the jet. Specifically, this proposal includes work on two approved cycle 14 Rossi X-ray Timing Explorer (RXTE) programs, an approved XMM-Newton program, as well as a synthesis study of transient black hole X-ray binaries using archival RXTE and radio data. We plan to use X-ray spectral and timing properties to determine the disk properties during the re-activation of the compact jet (as seen in the radio and infrared) during the decays of black hole transient outbursts, to determine how the inner disk properties change at low mass accretion rates, and to use RXTE along with multi-wavelength observations to constrain the jet properties required for the microquasar Cygnus~X-3 to produce high- energy emission. Due to the ubiquity of jets in astrophysical settings, these science topics are relevant to NASA programs dealing with the origin, structure, evolution, and destiny of the Universe, and especially to understanding phenomena near black holes.

  15. Accreting protoplanets in the LkCa 15 transition disk.

    PubMed

    Sallum, S; Follette, K B; Eisner, J A; Close, L M; Hinz, P; Kratter, K; Males, J; Skemer, A; Macintosh, B; Tuthill, P; Bailey, V; Defrère, D; Morzinski, K; Rodigas, T; Spalding, E; Vaz, A; Weinberger, A J

    2015-11-19

    Exoplanet detections have revolutionized astronomy, offering new insights into solar system architecture and planet demographics. While nearly 1,900 exoplanets have now been discovered and confirmed, none are still in the process of formation. Transition disks, protoplanetary disks with inner clearings best explained by the influence of accreting planets, are natural laboratories for the study of planet formation. Some transition disks show evidence for the presence of young planets in the form of disk asymmetries or infrared sources detected within their clearings, as in the case of LkCa 15 (refs 8, 9). Attempts to observe directly signatures of accretion onto protoplanets have hitherto proven unsuccessful. Here we report adaptive optics observations of LkCa 15 that probe within the disk clearing. With accurate source positions over multiple epochs spanning 2009-2015, we infer the presence of multiple companions on Keplerian orbits. We directly detect Hα emission from the innermost companion, LkCa 15 b, evincing hot (about 10,000 kelvin) gas falling deep into the potential well of an accreting protoplanet. PMID:26581290

  16. Accreting protoplanets in the LkCa 15 transition disk.

    PubMed

    Sallum, S; Follette, K B; Eisner, J A; Close, L M; Hinz, P; Kratter, K; Males, J; Skemer, A; Macintosh, B; Tuthill, P; Bailey, V; Defrère, D; Morzinski, K; Rodigas, T; Spalding, E; Vaz, A; Weinberger, A J

    2015-11-19

    Exoplanet detections have revolutionized astronomy, offering new insights into solar system architecture and planet demographics. While nearly 1,900 exoplanets have now been discovered and confirmed, none are still in the process of formation. Transition disks, protoplanetary disks with inner clearings best explained by the influence of accreting planets, are natural laboratories for the study of planet formation. Some transition disks show evidence for the presence of young planets in the form of disk asymmetries or infrared sources detected within their clearings, as in the case of LkCa 15 (refs 8, 9). Attempts to observe directly signatures of accretion onto protoplanets have hitherto proven unsuccessful. Here we report adaptive optics observations of LkCa 15 that probe within the disk clearing. With accurate source positions over multiple epochs spanning 2009-2015, we infer the presence of multiple companions on Keplerian orbits. We directly detect Hα emission from the innermost companion, LkCa 15 b, evincing hot (about 10,000 kelvin) gas falling deep into the potential well of an accreting protoplanet.

  17. Ultraviolet line diagnostics of accretion disk winds in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Vitello, Peter; Shlosman, Isaac

    1993-06-01

    The IUE data base is used to analyze the UV line shapes of the cataclysmic variables RW Sex, RW Tri, and V Sge. Observed lines are compared to synthetic line profiles computed using a model of rotating biconical winds from accretion disks. The wind model calculates the wind ionization structure self-consistently including photoionization from the disk and boundary layer and treats 3D line radiation transfer in the Sobolev approximation. It is found that winds from accretion disks provide a good fit for reasonable parameters to the observed UV lines which include the P Cygni profiles for low-inclination systems and pure emission at large inclination. Disk winds are preferable to spherical winds which originate on the white dwarf because they: (1) require a much lower ratio of mass-loss rate to accretion rate and are therefore more plausible energetically; (2) provide a natural source for a biconical distribution of mass outflow which produces strong scattering far above the disk leading to P Cygni profiles for low-inclination systems and pure line emission profiles at high inclination with the absence of eclipses in UV lines; and (3) produce rotation-broadened pure emission lines at high inclination.

  18. UV line diagnostics of accretion disk winds in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Vitello, Peter; Shlosman, Isaac

    1992-02-01

    The IUE data base is used to analyze the UV line shapes of cataclysmic variables RW Sex, RW Tri, and V Sge. Observed lines are compared to synthetic line profiles computed using a model of rotating bi-conical winds from accretion disks. The wind model calculates the wind ionization structure self-consistently including photoionization from the disk and boundary layer and treats 3-D line radiation transfer in the Sobolev approximation. It is found that winds from accretion disks provide a good fit for reasonable parameters to the observed UV lines which include the P Cygni profiles for low inclination systems and pure emission at large inclination. Disk winds are preferable to spherical winds which originate on the white dwarf because they (1) require a much lower ratio of mass loss rate to accretion rate and are therefore more plausible energetically, (2) provide a natural source for a bi-conical distribution of mass outflow which produces strong scattering far above the disk leading to P Cygni profiles for low inclination systems, and pure line emission profiles at high inclination with the absence of eclipses in UV lines, and (3) produce rotation broadened pure emission lines at high inclination.

  19. Isothermal, Compton-heated coronae above accretion disks

    NASA Technical Reports Server (NTRS)

    Ostriker, Eve C.; Mckee, Christopher F.; Klein, Richard I.

    1991-01-01

    The structure of Compton-heated coronae above accretion disks is studied here by using analytic and numerical approaches are used here to determine the direct and scattered radiation reaching the base of the corona for a range of central source luminosities. It is found that the outer region of the corona is unaffected by multiple scattering in the interior, provided that the luminosity of the central source is sufficient below the Eddington limit. How attenuation and scattering by the corona affects the strength of chromospheric emission lines is determined, as is the condition for which the irradiation due to the central source exceeds the locally generated flux from the disk. Finally, it is shown that the stability analysis for irradiated accretion disks of Tuchman et al. is not substantially altered by the corona.

  20. Electrodynamics of disk-accreting magnetic neutron stars

    NASA Technical Reports Server (NTRS)

    Miller, M. Coleman; Lamb, Frederick K.; Hamilton, Russell J.

    1994-01-01

    We have investigated the electrodynamics of magnetic neutron stars accreting from Keplerian disks and the implications for particle acceleration and gamma-ray emission by such systems. We argue that the particle density in the magnetospheres of such stars is larger by orders of magnitude than the Goldreich-Julian density, so that the formation of vacuum gaps is unlikely. We show that even if the star rotates slowly, electromotive forces (EMFs) of order 10(exp 15) V are produced by the interaction of plasma in the accretion disk with the magnetic field of the neutron star. The resistance of the disk-magnetosphere-star circuit is small, and hence these EMFs drive very large conduction currents. Such large currents are likely to produce magnetospheric instabilities, such as relativistic double layers and reconnection events, that can accelerate electrons or ions to very high energies.

  1. Self-gravity in neutrino-dominated accretion disks

    SciTech Connect

    Liu, Tong; Yu, Xiao-Fei; Gu, Wei-Min; Lu, Ju-Fu

    2014-08-10

    We present the effects of self-gravity on the vertical structure and neutrino luminosity of the neutrino-dominated accretion disks in cylindrical coordinates. It is found that significant changes of the structure appear in the outer region of the disk, especially for high accretion rates (e.g., ≳ 1 M{sub ☉} s{sup –1}), and thus cause the slight increase in the neutrino luminosity. Furthermore, the gravitational instability of the disk is reviewed by the vertical distribution of the Toomre parameter, which may account for the late-time flares in gamma-ray bursts and the extended emission in short-duration gamma-ray bursts.

  2. Exploring Stability of General Relativistic Accretion Disks

    NASA Astrophysics Data System (ADS)

    Korobkin, Oleg; Abdikamalov, Ernazar; Schnetter, Erik; Stergioulas, Nikolaos; Zink, Burkhard

    2011-04-01

    Self-gravitating relativistic disks around black holes can form as transient structures in a number of astrophysical scenarios, involving core collapse of massive stars and mergers of compact ob jects. I will present results on our recent study of the stability of such disks against runaway and non-axisymmetric instabilities, which we explore using three-dimensional hydrodynamics simulations in full general relativity. All of our models develop unstable non-axisymmetric modes on a dynamical timescale. We observe two distinct types of instabilities: the Papaloizou-Pringle and the so-called intermediate type instabilities. The development of the non-axisymmetric mode with azimuthal number m=1 is accompanied by an outspiraling motion of the black hole, which significantly amplifies the growth rate of the m=1 mode in some cases. We will discuss the types, growth rates and pattern speeds of the unstable modes, as well as the detectability of the gravitational waves from such objects.

  3. Gas accretion from halos to disks: observations, curiosities, and problems

    NASA Astrophysics Data System (ADS)

    Elmegreen, Bruce G.

    2016-08-01

    Accretion of gas from the cosmic web to galaxy halos and ultimately their disks is a prediction of modern cosmological models but is rarely observed directly or at the full rate expected from star formation. Here we illustrate possible large-scale cosmic HI accretion onto the nearby dwarf starburst galaxy IC10, observed with the VLA and GBT. We also suggest that cosmic accretion is the origin of sharp metallicity drops in the starburst regions of other dwarf galaxies, as observed with the 10-m GTC. Finally, we question the importance of cosmic accretion in normal dwarf irregulars, for which a recent study of their far-outer regions sees no need for, or evidence of, continuing gas buildup.

  4. Plasma Regimes in the Surroundings of Black Holes, Composite Plasma Disk Structures and Relevant Accretion Processes

    NASA Astrophysics Data System (ADS)

    Coppi, Bruno

    2010-06-01

    The theory of the composite plasma disk structures and of the relevant magnetic field configurations that can surround black holes is presented, consistently with recent experimental observations indicating that highly coherent magnetic field configurations exist in the core of these structures. Concepts developed to describe the physics of magnetically confined laboratory plasmas are used. Thus the ``paradox,'' that arises when considering accreting plasmas in the presence of a transverse magnetic field is resolved by considering accretion as an intermittent process whereby particles are carried in steps, along a sequence of magnetic separatrices containing the formed magnetic islands, by the onset of the equivalent of ``edge localized modes'' (ELMs) observed in laboratory experiments. Inactive galactic black holes are suggested as being associated with older galaxies that have been subjected to collisions destroying the coherent structures needed to guide relevant accretion flows. Alternatively, tridimensional spiral structures can emerge from axisymmetric disk configurations in a region close to the black hole and guide the relevant accretion flows. The radial gradient of the rotation frequency and the vertical gradient of the plasma pressure are the excitation factors for spirals as well as for axisymmetric modes. These can produce vertical flows of thermal energy and particles in opposing directions that can be connected to the winds emanating from disks in Active Galactic Nuclei (AGNs). In the close vicinity of Binary Black Holes the existence of three characteristic plasma regions is envisioned. The intermediate of these regions exhibits three physical regimes that differ both for the magnetic field structure and the spectrum of the emitted radiation, with jets and High Frequency Periodic Oscillations (HFQPOs) produced in two of these regimes.

  5. The self-regulated AGN feedback loop: the role of chaotic cold accretion

    NASA Astrophysics Data System (ADS)

    Gaspari, M.

    Supermassive black hole accretion and feedback play central role in the evolution of galaxies, groups, and clusters. I review how AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion (CCA). In a turbulent and heated atmosphere, cold clouds and kpc-scale filaments condense out of the plasma via thermal instability and rain toward the black hole. In the nucleus, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation or mixing, boosting the accretion rate up to 100 times the Bondi rate. The rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. The AGN heating stifles the formation of multiphase gas and accretion, the feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, CCA creates a symbiotic link between the black hole and the whole host via a tight self-regulated feedback which preserves the gaseous halo in global thermal equilibrium throughout cosmic time.

  6. X-ray Illuminated Ionized Skin of Accretion Disks

    NASA Astrophysics Data System (ADS)

    Nayakshin, S.

    Recently (Nayakshin, Kazanas & Kallman 2000), it was confirmed that the X-ray illuminated upper layer of accretion disk is unstable to the thermal ionization instability, and that most of the previous models of X-ray reflection and iron Kα line emission missed this instability. Not only the detailed predictions of these models, but even the intuition that one gained from the theoretical literature in the last 10 years is often misleading. We discuss how we can deduce the geometry of the primary X-ray source(s) by studying X-ray illuminated accretion disks. In particular, we show that the spectra and iron lines from disks that are illuminated by a single source above the black hole will usually contain highly ionized features that are rarely seen in real spectra. A full corona above a standard like disk will yield a neutral-like reflection component and iron Kα lines and is also incompatible with observations. On the other hand, if the X-ray sources are magnetic flares, then the X-ray flux illuminating the disk surface near to them is very much higher than the disk flux, leading to cold-like reflection and lines with an apparently smaller normalization. Therefore, we claim that the former model is strongly favored by the existing data.

  7. Thin accretion disks onto brane world black holes

    SciTech Connect

    Pun, C. S. J.; Harko, T.; Kovacs, Z.

    2008-10-15

    The brane-world description of our universe entails a large extra dimension and a fundamental scale of gravity that might be lower by several orders of magnitude as compared to the Planck scale. An interesting consequence of the brane-world scenario is in the nature of the vacuum solutions of the brane gravitational field equations, with properties quite distinct as compared to the standard black-hole solutions of general relativity. One possibility of observationally discriminating between different types of black holes is the study of the emission properties of the accretion disks. In the present paper we obtain the energy flux, the emission spectrum and accretion efficiency from the accretion disks around several classes of static and rotating brane-world black holes, and we compare them to the general relativistic case. Particular signatures can appear in the electromagnetic spectrum, thus leading to the possibility of directly testing extra-dimensional physical models by using astrophysical observations of the emission spectra from accretion disks.

  8. Dynamics of matter available for accretion in AGN

    NASA Astrophysics Data System (ADS)

    Proga, Daniel

    We propose to study fluid dynamics in the central regions of active galaxies. Several key attributes of AGN, such as infall rates onto the central black hole and the geometry and rates of outward energy and momentum transfer, are determined in their central regions. However, despite many years of intensive studies, how radiation and mass outflows affect the structure of active galaxies remains uncertain, mostly because multi- dimensional geometrical effects and the dynamics of gas and dust are poorly understood. We will construct time-dependent, multi-dimensional radiation hydrodynamical simulations to study, from first principles, the morphology of central galactic flows on scales ranging from subparsec to tens of parsecs. The novel features of our proposed models are: (1) radiative heating and cooling of dust and gas, (2) radiation pressure on dust and gas,
(3) the essential physics of dust sublimation, and (4) radiative transfer. The results of this project will reveal the basic physical properties of flows in AGN with unprecedented accuracy and detail. We will use our models to calculate diagnostics such as line profiles, broad band spectra, and intensity maps. These diagnostics will be compared with observations by both current NASA missions such as HST, Chandra, Spitzer, and Swift and planned missions such as JWST.

  9. Constraints on r-process nucleosynthesis in accretion disks

    NASA Technical Reports Server (NTRS)

    Jin, Liping

    1991-01-01

    Systems in which accretion drives an outflow from a region near a compact object may enrich the interstellar medium in r-process elements. A detailed assessment of the efficacy of this mechanism for the r-process is presented here, taking into account the constraints imposed by typical accretion-disk conditions. It is concluded that r-process elements are unlikely to have been made in this way, largely because the total production is too low, by a factor of about 100,000, to explain the observed abundances.

  10. Hot accretion disks with pairs: Effects of magnetic field and thermal cyclocsynchrotron radiation

    NASA Technical Reports Server (NTRS)

    Kusunose, Masaaki; Zdziarski, Andrzej A.

    1994-01-01

    We show the effects of thermal cyclosynchrotron radiation and magnetic viscosity on the structure of hot, two-temperature accretion disks. Magnetic field, B, is assumed to be randomly oriented and the ratio of magnetic pressure to either gas pressure, alpha = P(sub mag)/P(sub gas), or the sum of the gas and radiation pressures, alpha = (P(sub mag)/P(sub gas) + P(sub rad)), is fixed. We find those effects do not change the qualitative properties of the disks, i.e., there are still two critical accretion rates related to production of e(sup +/-) pairs, (M dot)((sup U)(sub cr)) and (M dot)((sup L)(sub cr)), that affect the number of local and global disk solutions, as recently found by Bjoernsson and Svensson for the case with B = 0. However, a critical value of the alpha-viscosity parameter above which those critical accretion rates disappear becomes smaller than alpha(sub cr) = 1 found in the case of B = 0, for P(sub mag) = alpha(P(sub gas) + P(sub rad)). If P(sub mag) = alpha P(sub gas), on the other hand, alpha(sub cr) is still about unity. Moreover, when Comptonized cyclosynchrotron radiation dominates Comptonized bremsstrahlung, radiation from the disk obeys a power law with the energy spectral index of approximately 0.5, in a qualitative agreement with X-ray observations of active galactic nuclei (AGNS) and Galactic black hole candidates. We also extend the hot disk solutions for P(sub mag) = alpha(P(sub gas) + P(sub rad)) to the effectively optically thick region, where they merge with the standard cold disk solutions. We find that the mapping method by Bjoernsson and Svensson gives a good approximation to the disk structure in the hot region and show where it breaks in the transition region. Finally, we find a region in the disk parameter space with no solutions due to the inability of Coulomb heating to supply enough energy to electrons.

  11. The average size and temperature profile of quasar accretion disks

    SciTech Connect

    Jiménez-Vicente, J.; Mediavilla, E.; Muñoz, J. A.; Motta, V.; Falco, E.

    2014-03-01

    We use multi-wavelength microlensing measurements of a sample of 10 image pairs from 8 lensed quasars to study the structure of their accretion disks. By using spectroscopy or narrowband photometry, we have been able to remove contamination from the weakly microlensed broad emission lines, extinction, and any uncertainties in the large-scale macro magnification of the lens model. We determine a maximum likelihood estimate for the exponent of the size versus wavelength scaling (r{sub s} ∝λ {sup p}, corresponding to a disk temperature profile of T∝r {sup –1/p}) of p=0.75{sub −0.2}{sup +0.2} and a Bayesian estimate of p = 0.8 ± 0.2, which are significantly smaller than the prediction of the thin disk theory (p = 4/3). We have also obtained a maximum likelihood estimate for the average quasar accretion disk size of r{sub s}=4.5{sub −1.2}{sup +1.5} lt-day at a rest frame wavelength of λ = 1026 Å for microlenses with a mean mass of M = 1 M {sub ☉}, in agreement with previous results, and larger than expected from thin disk theory.

  12. Visualizing SPH Cataclysmic Variable Accretion Disk Simulations with Blender

    NASA Astrophysics Data System (ADS)

    Kent, Brian R.; Wood, Matthew A.

    2015-01-01

    We present innovative ways to use Blender, a 3D graphics package, to visualize smoothed particle hydrodynamics particle data of cataclysmic variable accretion disks. We focus on the methods of shape key data constructs to increasedata i/o and manipulation speed. The implementation of the methods outlined allow for compositing of the various visualization layers into a final animation. The viewing of the disk in 3D from different angles can allow for a visual analysisof the physical system and orbits. The techniques have a wide ranging set of applications in astronomical visualization,including both observation and theoretical data.

  13. Irradiation Instability at the Inner Edges of Accretion Disks

    NASA Astrophysics Data System (ADS)

    Fung, Jeffrey; Artymowicz, Pawel

    2014-07-01

    An instability can potentially operate in highly irradiated disks where the disk sharply transitions from being radially transparent to opaque (the "transition region"). Such conditions may exist at the inner edges of transitional disks around T Tauri stars and accretion disks around active galactic nuclei. We derive the criterion for this instability, which we term the "irradiation instability," or IRI. We also present the linear growth rate as a function of β, the ratio between radiation force and gravity, and c s, the sound speed of the disk, obtained using two methods: a semi-analytic analysis of the linearized equations and a numerical simulation using the GPU-accelerated hydrodynamical code PEnGUIn. In particular, we find that IRI occurs at β ~ 0.1 if the transition region extends as wide as ~0.05r, and at higher β values if it is wider. This threshold value applies to c s ranging from 3% of the Keplerian orbital speed to 5%, and becomes higher if c s is lower. Furthermore, in the nonlinear evolution of the instability, disks with a large β and small c s exhibit "clumping," extreme local surface density enhancements that can reach over 10 times the initial disk surface density.

  14. Irradiation instability at the inner edges of accretion disks

    SciTech Connect

    Fung, Jeffrey; Artymowicz, Pawel

    2014-07-20

    An instability can potentially operate in highly irradiated disks where the disk sharply transitions from being radially transparent to opaque (the 'transition region'). Such conditions may exist at the inner edges of transitional disks around T Tauri stars and accretion disks around active galactic nuclei. We derive the criterion for this instability, which we term the 'irradiation instability', or IRI. We also present the linear growth rate as a function of β, the ratio between radiation force and gravity, and c{sub s}, the sound speed of the disk, obtained using two methods: a semi-analytic analysis of the linearized equations and a numerical simulation using the GPU-accelerated hydrodynamical code PEnGUIn. In particular, we find that IRI occurs at β ∼ 0.1 if the transition region extends as wide as ∼0.05r, and at higher β values if it is wider. This threshold value applies to c{sub s} ranging from 3% of the Keplerian orbital speed to 5%, and becomes higher if c{sub s} is lower. Furthermore, in the nonlinear evolution of the instability, disks with a large β and small c{sub s} exhibit 'clumping', extreme local surface density enhancements that can reach over 10 times the initial disk surface density.

  15. Fossil magnetic field of accretion disks of young stars

    NASA Astrophysics Data System (ADS)

    Dudorov, A. E.; Khaibrakhmanov, S. A.

    2014-07-01

    We elaborate the model of accretion disks of young stars with the fossil large-scale magnetic field in the frame of Shakura and Sunyaev approximation. Equations of the MHD model include Shakura and Sunyaev equations, induction equation and equations of ionization balance. Magnetic field is determined taking into account ohmic diffusion, magnetic ambipolar diffusion and buoyancy. Ionization fraction is calculated considering ionization by cosmic rays and X-rays, thermal ionization, radiative recombinations and recombinations on the dust grains. Analytical solution and numerical investigations show that the magnetic field is coupled to the gas in the case of radiative recombinations. Magnetic field is quasi-azimuthal close to accretion disk inner boundary and quasi-radial in the outer regions. Magnetic field is quasi-poloidal in the dusty "dead" zones with low ionization degree, where ohmic diffusion is efficient. Magnetic ambipolar diffusion reduces vertical magnetic field in 10 times comparing to the frozen-in field in this region. Magnetic field is quasi-azimuthal close to the outer boundary of accretion disks for standard ionization rates and dust grain size a d=0.1 μm. In the case of large dust grains ( a d>0.1 μm) or enhanced ionization rates, the magnetic field is quasi-radial in the outer regions. It is shown that the inner boundary of dusty "dead" zone is placed at r=(0.1-0.6) AU for accretion disks of stars with M=(0.5-2) M ⊙. Outer boundary of "dead" zone is placed at r=(3-21) AU and it is determined by magnetic ambipolar diffusion. Mass of solid material in the "dead" zone is more than 3 M ⊕ for stars with M≥1 M ⊙.

  16. CONTRIBUTION OF THE ACCRETION DISK, HOT CORONA, AND OBSCURING TORUS TO THE LUMINOSITY OF SEYFERT GALAXIES: INTEGRAL AND SPITZER OBSERVATIONS

    SciTech Connect

    Sazonov, S.; Churazov, E.; Krivonos, R.; Revnivtsev, M.; Sunyaev, R.; Vikhlinin, A.; Hickox, R. C.; Gorjian, V.; Werner, M. W.; Fabian, A. C.; Forman, W. R.

    2012-10-01

    We estimate the relative contributions of the supermassive black hole (SMBH) accretion disk, corona, and obscuring torus to the bolometric luminosity of Seyfert galaxies, using Spitzer mid-infrared (MIR) observations of a complete sample of 68 nearby active galactic nuclei (AGNs) from the INTEGRAL all-sky hard X-ray (HX) survey. This is the first HX-selected (above 15 keV) sample of AGNs with complementary high angular resolution, high signal-to-noise, MIR data. Correcting for the host galaxy contribution, we find a correlation between HX and MIR luminosities: L{sub 15{mu}m}{proportional_to}L{sup 0.74{+-}0.06}{sub HX}. Assuming that the observed MIR emission is radiation from an accretion disk reprocessed in a surrounding dusty torus that subtends a solid angle decreasing with increasing luminosity (as inferred from the declining fraction of obscured AGNs), the intrinsic disk luminosity, L{sub Disk}, is approximately proportional to the luminosity of the corona in the 2-300 keV energy band, L{sub Corona}, with the L{sub Disk}/L{sub Corona} ratio varying by a factor of 2.1 around a mean value of 1.6. This ratio is a factor of {approx}2 smaller than for typical quasars producing the cosmic X-ray background. Therefore, over three orders of magnitude in luminosity, HX radiation carries a large, and roughly comparable, fraction of the bolometric output of AGNs. We estimate the cumulative bolometric luminosity density of local AGNs at {approx}(1-3) Multiplication-Sign 10{sup 40} erg s{sup -1} Mpc{sup -3}. Finally, the Compton temperature ranges between kT{sub c} Almost-Equal-To 2 and Almost-Equal-To 6 keV for nearby AGNs, compared to kT{sub c} Almost-Equal-To 2 keV for typical quasars, confirming that radiative heating of interstellar gas can play an important role in regulating SMBH growth.

  17. Gravitomagnetic Acceleration of Black Hole Accretion Disk Matter to Polar Jets

    NASA Astrophysics Data System (ADS)

    Poirier, John; Mathews, Grant

    2015-04-01

    It is shown that the motion of the neutral masses in an accretion disk orbiting a black hole creates a magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near the accretion disk away from the disk and then inward toward the axis of the accretion disk. Moreover, as the accelerated material nears the axis, a frame-dragging effect twists the trajectories around the axis thus contributing to the formation of a narrow polar jet emanating from the poles.

  18. MAGNETIZED ACCRETION AND DEAD ZONES IN PROTOSTELLAR DISKS

    SciTech Connect

    Dzyurkevich, Natalia; Henning, Thomas; Turner, Neal J.; Kley, Wilhelm

    2013-03-10

    The edges of magnetically dead zones in protostellar disks have been proposed as locations where density bumps may arise, trapping planetesimals and helping form planets. Magneto-rotational turbulence in magnetically active zones provides both accretion of gas on the star and transport of mass to the dead zone. We investigate the location of the magnetically active regions in a protostellar disk around a solar-type star, varying the disk temperature, surface density profile, and dust-to-gas ratio. We also consider stellar masses between 0.4 and 2 M{sub Sun }, with corresponding adjustments in the disk mass and temperature. The dead zone's size and shape are found using the Elsasser number criterion with conductivities including the contributions from ions, electrons, and charged fractal dust aggregates. The charged species' abundances are found using the approach proposed by Okuzumi. The dead zone is in most cases defined by the ambipolar diffusion. In our maps, the dead zone takes a variety of shapes, including a fish tail pointing away from the star and islands located on and off the midplane. The corresponding accretion rates vary with radius, indicating locations where the surface density will increase over time, and others where it will decrease. We show that density bumps do not readily grow near the dead zone's outer edge, independently of the disk parameters and the dust properties. Instead, the accretion rate peaks at the radius where the gas-phase metals freeze out. This could lead to clearing a valley in the surface density, and to a trap for pebbles located just outside the metal freezeout line.

  19. CHEMICAL EVOLUTION OF PROTOPLANETARY DISKS-THE EFFECTS OF VISCOUS ACCRETION, TURBULENT MIXING, AND DISK WINDS

    SciTech Connect

    Heinzeller, D.; Nomura, H.; Walsh, C.; Millar, T. J.

    2011-04-20

    We calculate the chemical evolution of protoplanetary disks considering radial viscous accretion, vertical turbulent mixing, and vertical disk winds. We study the effects on the disk chemical structure when different models for the formation of molecular hydrogen on dust grains are adopted. Our gas-phase chemistry is extracted from the UMIST Database for Astrochemistry (Rate06) to which we have added detailed gas-grain interactions. We use our chemical model results to generate synthetic near- and mid-infrared local thermodynamic equilibrium line emission spectra and compare these with recent Spitzer observations. Our results show that if H{sub 2} formation on warm grains is taken into consideration, the H{sub 2}O and OH abundances in the disk surface increase significantly. We find that the radial accretion flow strongly influences the molecular abundances, with those in the cold midplane layers particularly affected. On the other hand, we show that diffusive turbulent mixing affects the disk chemistry in the warm molecular layers, influencing the line emission from the disk and subsequently improving agreement with observations. We find that NH{sub 3}, CH{sub 3}OH, C{sub 2}H{sub 2}, and sulfur-containing species are greatly enhanced by the inclusion of turbulent mixing. We demonstrate that disk winds potentially affect the disk chemistry and the resulting molecular line emission in a manner similar to that found when mixing is included.

  20. Structure of relativistic accretion disk with non-standard model

    NASA Astrophysics Data System (ADS)

    Khesali, A. R.; Salahshoor, K.

    2016-07-01

    The structure of stationary, axisymmetric advection-dominated accretion disk (ADAF) around rotating black hole, using non-standard model, was examined. In this model, the transport efficiency of the angular momentum α was dependent on the magnetic Prandtl number α ∝ Pm^{δ } . The full relativistic shear stress recently obtained by a new manner, was used. By considering black hole spin and Prandtl number instantaneously, the structure of ADAFs was changed in inner and outer region of the disk. It was discovered that the accretion flow was denser and hotter in the inner region, due to the black hole spin, and in the outer region, due to the presence of Prandtl parameter. Inasmuch as the rotation of the black hole affected the transport efficiency of angular momentum in parts of the disk very close to the even horizon, then in these regions, the viscosity depended on the rotation of black hole. Also, it was discovered that the effect of the black hole spin on the structure of the disk was related to the presence of Prandtl parameter.

  1. Thin Disks Gone MAD: Magnetically Arrested Accretion in the Thin Regime

    NASA Astrophysics Data System (ADS)

    Avara, Mark J.; McKinney, Jonathan C.; Reynolds, Christopher S.

    2015-01-01

    The collection and concentration of surrounding large scale magnetic fields by black hole accretion disks may be required for production of powerful, spin driven jets. So far, accretion disks have not been shown to grow sufficient poloidal flux via the turbulent dynamo alone to produce such persistent jets. Also, there have been conflicting answers as to how, or even if, an accretion disk can collect enough magnetic flux from the ambient environment. Extending prior numerical studies of magnetically arrested disks (MAD) in the thick (angular height, H/R~1) and intermediate (H/R~.2-.6) accretion regimes, we present our latest results from fully general relativistic MHD simulations of the thinnest BH (H/R~.1) accretion disks to date exhibiting the MAD mode of accretion. We explore the significant deviations of this accretion mode from the standard picture of thin, MRI-driven accretion, and demonstrate the accumulation of large-scale magnetic flux.

  2. Formation and evolution of inclined accretion disks in intermediate polars

    NASA Astrophysics Data System (ADS)

    Fateeva, A. M.; Zhilkin, A. G.; Bisikalo, D. V.

    2016-01-01

    The results of 3D modeling of the formation of the accretion disks of intermediate polars are presented. A model with misaligned rotation axes of accretor and the orbit is onsidered, in which it is assumed that the white dwarf has a dipolar magnetic field with its symmetry axis inclined to the whitedwarf rotation and orbital axes. The computations show that, in the early stages of formation of the disk, the action of magnetic field is able to create the initial (seed) inclination of the disk. This inclination is then supported mainly by the dynamical pressure of the flow from the inner Lagrangian point L1. As themass of the disk increases, the inclination disappears. Under certain conditions, the disk inclination does not arise in systems with misaligned white-dwarf rotation and orbital axes. The influence of the magnetic field and asynchronous rotation of the accretor may result in the formation of spiral waves in the disk with amplitudes sufficient to be detected observationally.

  3. ULX Ngc 5408 X-1: : Coupled Spectral-timing Behavior Similar To Accreting Galactic Bhs And Agn ?

    NASA Astrophysics Data System (ADS)

    pasham, dheeraj r.; Strohmayer, T. E.

    2011-09-01

    The ultraluminous X-ray source (ULX) NGC 5408 X-1 is unique. It is one of the only few ULXs known to show quasi-periodic X-ray variability. The qualitative nature of its X-ray variability is consistent with stellar mass black holes in the Steep Power-law (SPL) state; where the power spectral density rises below a certain frequency, then flattens and often shows a quasi periodic oscillation (QPO) on the rising portion. However, the characteristic frequencies seen in X-1 are scaled down by a factor of ˜ 100. Further, the soft X-ray excess (0.2 - 2.0 keV) present in the X-ray spectrum can be modeled with a disk black body, perhaps suggesting the presence of a cool disk (˜ 0.15 keV). Simple scaling of the characteristic frequencies noted in the power spectra, and scaling of the disk temperature imply a mass of ˜ > few 100 M ⊙ for the black hole. Hence, X-1 is a strong candidate for an intermediate mass black hole (100 - a few 1000 M⊙). However, this mass estimate is sensitive to the assumption that X-1 is in the SPL state, and is further complicated by the fact that the disk temperature scaling is applicable for systems whose total flux is dominated by emission from the disk. We present results from 700 ksecs of XMM Newton observations of X-1, including 420 ksecs of new exposure, which further explore the timing and spectral properties of X-1 and allow additional tests of the assumption that X-1 accretes like an SPL state black hole. We also present our results in the context of previously established "variability plane" correlations for both Galactic black holes and AGN. We discuss the implications of our results on mass estimates for X-1.

  4. THE QUASAR ACCRETION DISK SIZE-BLACK HOLE MASS RELATION

    SciTech Connect

    Morgan, Christopher W.; Kochanek, C. S.; Morgan, Nicholas D.; Falco, Emilio E. E-mail: ckochanek@astronomy.ohio-state.ed E-mail: efalco@cfa.harvard.ed

    2010-04-01

    We use the microlensing variability observed for 11 gravitationally lensed quasars to show that the accretion disk size at a rest-frame wavelength of 2500 A is related to the black hole mass by log(R{sub 2500}/cm) = (15.78 +- 0.12) + (0.80 +- 0.17)log(M{sub BH}/10{sup 9} M{sub sun}). This scaling is consistent with the expectation from thin-disk theory (R {proportional_to} M {sup 2/3}{sub BH}), but when interpreted in terms of the standard thin-disk model (T {proportional_to} R {sup -3/4}), it implies that black holes radiate with very low efficiency, log(eta) = -1.77 +- 0.29 + log(L/L{sub E}), where eta=L/(M-dot c{sup 2}). Only by making the maximum reasonable shifts in the average inclination, Eddington factors, and black hole masses can we raise the efficiency estimate to be marginally consistent with typical efficiency estimates (eta {approx} 10%). With one exception, these sizes are larger by a factor of {approx}4 than the size needed to produce the observed 0.8 {mu}m quasar flux by thermal radiation from a thin disk with the same T {proportional_to} R {sup -3/4} temperature profile. While scattering a significant fraction of the disk emission on large scales or including a large fraction of contaminating line emission can reduce the size discrepancy, resolving it also appears to require that accretion disks have flatter temperature/surface brightness profiles.

  5. Asymmetric evolution of magnetic reconnection in collisionless accretion disk

    SciTech Connect

    Shirakawa, Keisuke Hoshino, Masahiro

    2014-05-15

    An evolution of a magnetic reconnection in a collisionless accretion disk is investigated using a 2.5 dimensional hybrid code simulation. In astrophysical disks, magnetorotational instability (MRI) is considered to play an important role by generating turbulence in the disk and contributes to an effective angular momentum transport through a turbulent viscosity. Magnetic reconnection, on the other hand, also plays an important role on the evolution of the disk through a dissipation of a magnetic field enhanced by a dynamo effect of MRI. In this study, we developed a hybrid code to calculate an evolution of a differentially rotating system. With this code, we first confirmed a linear growth of MRI. We also investigated a behavior of a particular structure of a current sheet, which would exist in the turbulence in the disk. From the calculation of the magnetic reconnection, we found an asymmetric structure in the out-of-plane magnetic field during the evolution of reconnection, which can be understood by a coupling of the Hall effect and the differential rotation. We also found a migration of X-point whose direction is determined only by an initial sign of J{sub 0}×Ω{sub 0}, where J{sub 0} is the initial current density in the neutral sheet and Ω{sub 0} is the rotational vector of the background Keplerian rotation. Associated with the migration of X-point, we also found a significant enhancement of the perpendicular magnetic field compared to an ordinary MRI. MRI-Magnetic reconnection coupling and the resulting magnetic field enhancement can be an effective process to sustain a strong turbulence in the accretion disk and to a transport of angular momentum.

  6. The Jet/Disk Connection in AGN: Chandra and XMM-Newton Observations of Three Powerful Radio-Loud Quasars

    NASA Technical Reports Server (NTRS)

    Sambruna, Rita; Gliozzi, Mario; Tavecchio, F.; Maraschi, L.; Foschini, Luigi

    2007-01-01

    The connection between the accretion process that powers AGN and the formation of jets is still poorly understood. Here we tackle this issue using new, deep Chandra and XMM-Newton observations of tlie cores of three powerful radio loud quasars: 1136-135, 1150+497 (Chandra), and 0723+679 (XMM-Newton), in the redshift range z=0.3-0.8. These sources are known from our previous Chandra siiapsliot survey to liave kpc-scale X-ray jets. In 1136-135 and 1150-1+497; evidence is found for the presence of diffuse thermal X-ray emission around the cores; on scales of 40-50 kpc and with luminosity L(sub 0.3-2 kev approx. 10(sup 43) erg per second, suggesting thermal emission from the host galaxy or a galaxy group. The X-ray continua of the cores in the three sources are described by an upward-curved (concave) broken power law, with photon indices GAMMA (sub soft) approx. 1.8 - 2.1 and GAMMA (sub hard) approx. 1.7 below and above approx. equal to 2 keV, respectively. There is evidence for an uiiresolved Fe K alpha line with EW approx. 70 eV in the three quasars. The Spectral Energy Distributions of the sources can be well described by a mix of jet and disk emission, with the jet dominating the radio and hard X-rays (via synchrotron and external Compton) and the disk dominating the optical/UV through soft X-rays. The ratio of the jet-to-disk powers is approx. 1, consistent with those derived for a number of gamma ray emitting blazars. This indicates that near equality of accretion and jet power may be common in powerful radio-loud AGN.

  7. Buoyancy-limited magnetic viscosity in quasi-stellar object accretion disk models

    NASA Technical Reports Server (NTRS)

    Sakimoto, Philip J.; Coroniti, Ferdinand V.

    1989-01-01

    Alpha models of astrophysical accretion disks assume an ad hoc viscous stress which scales locally as the total pressure. It is shown here that, if turbulent magnetic Maxwell stresses are the source of this viscosity in quasi-stellar object accretion disk models, then the stress cannot follow the assumed alpha-law in the radiation pressure-dominated inner region of the disk.

  8. NUSTAR and SUZAKU X-ray spectroscopy of NGC 4151: Evidence for reflection from the inner accretion disk

    DOE PAGES

    Keck, M. L.; Brenneman, L. W.; Ballantyne, D. R.; Bauer, F.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Dauser, T.; Elvis, M.; Fabian, A. C.; et al

    2015-06-15

    We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spinmore » $$a\\gt 0.9$$ accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. As a result, we discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features.« less

  9. Emission of gravitational waves by precession of slim accretion disks dynamically driven by the Bardeen-Petterson effect

    NASA Astrophysics Data System (ADS)

    Alfonso, W. D.; Sánchez, L. A.; Mosquera, H. J.

    2015-11-01

    The electromagnetic radiation emitted from some astrophysical objects such as active galactic nuclei (AGN), micro-quasars (M-QSRs), and central engines of gamma-ray burst (GRBs), seems to have a similar physical origin: a powerful jet of plasma ejected from a localized system, presumably composed of an accretion disk encircling a compact object. This radiation is generally beamed in the polar directions and in some cases, it appears to have a spiral-like structure that could be explained if the central system itself precesses. In this work, we use the slim disk accretion model, presented by Popham et al. (1999), to studying the gravitational waves (GWs) emitted by the precession of the accretion disk around a solar-mass Kerr black hole (KBH). For practical purposes, this model describes the central engine of a class of GRBs when some astrophysical constrains are fulfilled. The induced precession considered here is driven by the Bardeen-Petterson effect, which results from the combination of viscous effects in such disks and the relativistic frame-dragging effect. We evaluate the feasibility of direct detection of the GWs computed for such a model and show that the precession of this kind of systems could be detected by gravitational wave observatories like DECIGO, ultimate-DECIGO, and BBO, with higher probability if such a class of sources are placed at distances less than 1 Mpc.

  10. Thin accretion disks in f(R) modified gravity models

    SciTech Connect

    Pun, C. S. J.; Harko, T.; Kovacs, Z.

    2008-07-15

    We consider the basic physical properties of matter forming a thin accretion disc in the static and spherically symmetric space-time metric of the vacuum f(R) modified gravity models. The Lagrangian of the generalized gravity theory is also obtained in a parametric form, and the conditions of the viability of the model are also discussed. The exact Schwarzschild-type solution of the gravitational field equations in the f(R) gravity contains a linearly increasing term, as well as a logarithmic correction, as compared to the standard Schwarzschild solution of general relativity, and it depends on four arbitrary integration constants. The energy flux and the emission spectrum from the accretion disk around the f(R) gravity black holes are obtained, and they are compared to the general relativistic case. Particular signatures can appear in the electromagnetic spectrum, thus leading to the possibility of directly testing modified gravity models by using astrophysical observations of the emission spectra from accretion disks.

  11. THE PHOTOIONIZED ACCRETION DISK IN HER X-1

    SciTech Connect

    Ji, L.; Schulz, N.; Nowak, M.; Marshall, H. L.; Kallman, T.

    2009-08-01

    We present an analysis of several high-resolution Chandra grating observations of the X-ray binary pulsar Her X-1. With a total exposure of 170 ks, the observations are separated by years and cover three combinations of orbital and superorbital phases. Our goal is to determine distinct properties of the photoionized emission and its dependence on phase-dependent variations of the continuum. We find that the continua can be described by a partial covering model which above 2 keV is consistent with recent results from Rossi X-Ray Timing Explorer studies and at low energies is consistent with recent XMM-Newton and BeppoSAX studies. Besides a power law with fixed index, an additional thermal blackbody of 114 eV is required to fit wavelengths above 12 A ({approx}1 keV). We find that likely all the variability is caused by highly variable absorption columns in the range (1-3) x 10{sup 23} cm{sup -2}. Strong Fe K line fluorescence in almost all observations reveals that dense, cool material is present not only in the outer regions of the disk but interspersed throughout the disk. Most spectra show strong line emission stemming from a photoionized accretion disk corona (ADC). We model the line emission with generic thermal plasma models as well as with the photoionization code XSTAR and investigate changes of the ionization balance with orbital and superorbital phases. Most accretion disk coronal properties such as disk radii, temperatures, and plasma densities are consistent with previous findings for the low state. We find that these properties change negligibly with respect to orbital and superorbital phases. A couple of the higher energy lines exhibit emissivities that are significantly in excess of expectations from a static ADC.

  12. ACCRETION RATES OF MOONLETS EMBEDDED IN CIRCUMPLANETARY PARTICLE DISKS

    SciTech Connect

    Ohtsuki, Keiji; Yasui, Yuki; Daisaka, Hiroshi

    2013-08-01

    We examine the gravitational capture probability of colliding particles in circumplanetary particle disks and accretion rates of small particles onto an embedded moonlet, using analytic calculation, three-body orbital integrations, and N-body simulations. Expanding our previous work, we take into account the Rayleigh distribution of particles' orbital eccentricities and inclinations in our analytic calculation and orbital integration and confirm agreement between them when the particle velocity dispersion is comparable to or larger than their mutual escape velocity and the ratio of the sum of the physical radii of colliding particles to their mutual Hill radius (r-tilde{sub p}) is much smaller than unity. As shown by our previous work, the capture probability decreases significantly when the velocity dispersion is larger than the escape velocity and/or r-tilde{sub p}{approx}>0.7. Rough surfaces of particles can enhance the capture probability. We compare the results of three-body calculations with N-body simulations for accretion of small particles by an embedded moonlet and find agreement at the initial stage of accretion. However, when particles forming an aggregate on the moonlet surface nearly fill the Hill sphere, the aggregate reaches a quasi-steady state with a nearly constant number of particles covering the moonlet, and the accretion rate is significantly reduced compared to the three-body results.

  13. ENHANCED ACCRETION RATES OF STARS ON SUPERMASSIVE BLACK HOLES BY STAR-DISK INTERACTIONS IN GALACTIC NUCLEI

    SciTech Connect

    Just, Andreas; Yurin, Denis; Makukov, Maxim; Berczik, Peter; Omarov, Chingis; Spurzem, Rainer; Vilkoviskij, Emmanuil Y.

    2012-10-10

    We investigate the dynamical interaction of a central star cluster surrounding a supermassive black hole (SMBH) and a central accretion disk (AD). The dissipative force acting on stars in the disk leads to an enhanced mass flow toward the SMBH and to an asymmetry in the phase space distribution due to the rotating AD. The AD is considered as a stationary Keplerian rotating disk, which is vertically extended in order to employ a fully self-consistent treatment of stellar dynamics including the dissipative force originating from star-gas ram pressure effects. The stellar system is treated with a direct high-accuracy N-body integration code. A star-by-star representation, desirable in N-body simulations, cannot be extended to real particle numbers yet. Hence, we carefully discuss the scaling behavior of our model with regard to particle number and tidal accretion radius. The main idea is to find a family of models for which the ratio of two-body relaxation time and dissipation time (for kinetic energy of stellar orbits) is constant, which then allows us to extrapolate our results to real parameters of galactic nuclei. Our model is derived from basic physical principles and as such it provides insight into the role of physical processes in galactic nuclei, but it should be regarded as a first step toward more realistic and more comprehensive simulations. Nevertheless, the following conclusions appear to be robust: the star accretion rate onto the AD and subsequently onto the SMBH is enhanced by a significant factor compared to purely stellar dynamical systems neglecting the disk. This process leads to enhanced fueling of central disks in active galactic nuclei (AGNs) and to an enhanced rate of tidal stellar disruptions. Such disruptions may produce electromagnetic counterparts in the form of observable X-ray flares. Our models improve predictions for their rates in quiescent galactic nuclei. We do not yet model direct stellar collisions in the gravitational potential

  14. Herbig Ae/Be stars - Intermediate-mass stars surrounded by massive circumstellar accretion disks

    NASA Technical Reports Server (NTRS)

    Hillenbrand, Lynne A.; Strom, Stephen E.; Vrba, Frederick J.; Keene, Jocelyn

    1992-01-01

    The proposition that Herbig Ae/Be stars are young intermediate mass stars surrounded by optically thick accretion disks is explored. From a study of 47 such objects, a subset of 30 stars is identified whose spectral energy distributions can be interpreted convincingly in terms of pre-main sequence stars surrounded by massive optically thick circumstellar accretion disks. Constraints on the physical properties of the disks, such as size, mass, accretion rate, lifetime, and radial structure are derived from the photometric data.

  15. Quiescent accretion disks in black hole X-ray novae

    NASA Technical Reports Server (NTRS)

    Orosz, Jerome A.; Bailyn, Charles D.; Remillard, Ronald A.; Mcclintock, Jeffrey E.; Foltz, Craig B.

    1994-01-01

    We present detailed time-resolved spectroscopy of the Balmer emission lines from two black hole binary systems in quiescence, A0620-00 and Nova Muscae 1991. We find extraordinary similarities between the two systems. There are 30-40 km/s velocity variations of the emission lines over the orbital period, the phases of which are not aligned with the expected phase of the motion of the compact primary. Detailed modeling of both systems is complicated by variable hot spot components, regions of optical thickness, and intermittent excess emission in the blue line wings of the H-alpha lines. Both sources also display low velocities at the outer edge of the accretion disk, implying a large primary Roche lobe and extreme mass ratios. These complications suggest that although simple optically thin, Keplerian alpha-disk models provide a useful parameterization of emission lines from these systems, the straightforward physical models they imply should be treated with great caution.

  16. Nucleosynthesis in Outflows from Kerr Black Hole Accretion Disks

    SciTech Connect

    Surman, Rebecca; Mclaughlin, Gail C; Sabbatino, N.; Hix, William Raphael

    2008-01-01

    Observational evidence suggests long-duration, collapsar-type gamma-ray bursts (GRBs) may have been more common in the early universe. These events are thought to be powered by accretion disks around black holes (AD-BHs). While GRBs require AD-BHs, not all AD-BHs will power bursts and therefore their formation will be more common than GRBs themselves. Outflows from AD-BHs may therefore be important contributors to the nuclear abundances in the oldest stars, particularly for rare species or those not uniformly observed. Here we consider the nucleosynthesis in this environment starting with relativistic disk models of Kerr black holes and following the element synthesis in a parameterized outflow. We present nucleosynthesis results from a range of AD-BH models.

  17. Black hole spin influence on accretion disk neutrino detection

    NASA Astrophysics Data System (ADS)

    Caballero, O. L.; Zielinski, T.; McLaughlin, G. C.; Surman, R.

    2016-06-01

    Neutrinos are copiously emitted from neutrino-cooled black hole accretion disks playing a fundamental role in their evolution, as well as in the production of gamma ray bursts and r-process nucleosynthesis. The black hole generates a strong gravitational field able to change the properties of the emerging neutrinos. We study the influence of the black hole spin on the structure of the neutrino surfaces, neutrino luminosities, average neutrino energies, and event counts at SuperK. We consider several disk models and provide estimates that cover different black hole efficiency scenarios. We discuss the influence of the detector's inclination with respect to the axis of the torus on neutrino properties. We find that tori around spinning black holes have larger luminosities, energies, and rates compared to tori around static black holes and that the inclination of the observer causes a reduction in the luminosities and detection rates but an increase in the average energies.

  18. The Anomalous Accretion Disk of the Cataclysmic Variable RW Sextantis

    NASA Astrophysics Data System (ADS)

    Linnell, Albert P.; Godon, P.; Hubeny, I.; Sion, E. M.; Szkody, P.

    2011-01-01

    The standard model for stable Cataclysmic Variable (CV) accretion disks (Frank, King and Raine 1992) derives an explicit analytic expression for the disk effective temperature as function of radial distance from the white dwarf (WD). That model specifies that the effective temperature, Teff(R), varies with R as ()0.25, where () represents a combination of parameters including R, the mass transfer rate M(dot), and other parameters. It is well known that fits of standard model synthetic spectra to observed CV spectra find almost no instances of agreement. We have derived a generalized expression for the radial temperature gradient, which preserves the total disk luminosity as function of M(dot) but permits a different exponent from the theoretical value of 0.25, and have applied it to RW Sex (Linnell et al.,2010,ApJ, 719,271). We find an excellent fit to observed FUSE and IUE spectra for an exponent of 0.125, curiously close to 1/2 the theoretical value. Our annulus synthetic spectra, combined to represent the accretion disk, were produced with program TLUSTY, were non-LTE and included H, He, C, Mg, Al, Si, and Fe as explicit ions. We illustrate our results with a plot showing the failure to fit RW Sex for a range of M(dot) values, our model fit to the observations, and a chi2 plot showing the selection of the exponent 0.125 as the best fit for the M(dot) range shown. (For the final model parameters see the paper cited.)

  19. Effects of Local Dissipation Profiles on Magnetized Accretion Disk Spectra

    NASA Astrophysics Data System (ADS)

    Tao, Ted; Blaes, Omer

    2013-06-01

    We present spectral calculations of non-LTE accretion disk models appropriate for high-luminosity stellar mass black hole X-ray binary systems. We first use a dissipation profile based on scaling the results of shearing box simulations of Hirose et al. to a range of annuli parameters. We simultaneously scale the effective temperature, orbital frequency, and surface density with luminosity and radius according to the standard α-model. This naturally brings increased dissipation to the disk surface layers (around the photospheres) at small radii and high luminosities. We find that the local spectrum transitions directly from a modified blackbody to a saturated Compton scattering spectrum as we increase the effective temperature and orbital frequency while decreasing midplane surface density. Next, we construct annuli models based on the parameters of a L/L Edd = 0.8 disk orbiting a 6.62 solar mass black hole using two modified dissipation profiles that explicitly put more dissipation per unit mass near the disk surface. The new dissipation profiles are qualitatively similar to the one found by Hirose et al., but produce strong near power-law spectral tails. Our models also include physically motivated magnetic acceleration support based once again on scaling the Hirose et al. results. We present three full-disk spectra, each based on one of the dissipation prescriptions. Our most aggressive dissipation profile results in a disk spectrum that is in approximate quantitative agreement with certain observations of the steep power-law spectral states from some black hole X-ray binaries.

  20. EFFECTS OF LOCAL DISSIPATION PROFILES ON MAGNETIZED ACCRETION DISK SPECTRA

    SciTech Connect

    Tao, Ted; Blaes, Omer

    2013-06-10

    We present spectral calculations of non-LTE accretion disk models appropriate for high-luminosity stellar mass black hole X-ray binary systems. We first use a dissipation profile based on scaling the results of shearing box simulations of Hirose et al. to a range of annuli parameters. We simultaneously scale the effective temperature, orbital frequency, and surface density with luminosity and radius according to the standard {alpha}-model. This naturally brings increased dissipation to the disk surface layers (around the photospheres) at small radii and high luminosities. We find that the local spectrum transitions directly from a modified blackbody to a saturated Compton scattering spectrum as we increase the effective temperature and orbital frequency while decreasing midplane surface density. Next, we construct annuli models based on the parameters of a L/L{sub Edd} = 0.8 disk orbiting a 6.62 solar mass black hole using two modified dissipation profiles that explicitly put more dissipation per unit mass near the disk surface. The new dissipation profiles are qualitatively similar to the one found by Hirose et al., but produce strong near power-law spectral tails. Our models also include physically motivated magnetic acceleration support based once again on scaling the Hirose et al. results. We present three full-disk spectra, each based on one of the dissipation prescriptions. Our most aggressive dissipation profile results in a disk spectrum that is in approximate quantitative agreement with certain observations of the steep power-law spectral states from some black hole X-ray binaries.

  1. Magnetized Accretion-Ejection Structures: 2.5-dimensional Magnetohydrodynamic Simulations of Continuous Ideal Jet Launching from Resistive Accretion Disks

    NASA Astrophysics Data System (ADS)

    Casse, Fabien; Keppens, Rony

    2002-12-01

    We present numerical magnetohydrodynamic (MHD) simulations of a magnetized accretion disk launching trans-Alfvénic jets. These simulations, performed in a 2.5-dimensional time-dependent polytropic resistive MHD framework, model a resistive accretion disk threaded by an initial vertical magnetic field. The resistivity is only important inside the disk and is prescribed as η=αmVAHexp(- 2Z2/H2), where VA stands for Alfvén speed, H is the disk scale height, and the coefficient αm is smaller than unity. By performing the simulations over several tens of dynamical disk timescales, we show that the launching of a collimated outflow occurs self-consistently and the ejection of matter is continuous and quasi-stationary. These are the first ever simulations of resistive accretion disks launching nontransient ideal MHD jets. Roughly 15% of accreted mass is persistently ejected. This outflow is safely characterized as a jet since the flow becomes superfast magnetosonic, well collimated, and reaches a quasi-stationary state. We present a complete illustration and explanation of the ``accretion-ejection'' mechanism that leads to jet formation from a magnetized accretion disk. In particular, the magnetic torque inside the disk brakes the matter azimuthally and allows for accretion, while it is responsible for an effective magnetocentrifugal acceleration in the jet. As such, the magnetic field channels the disk angular momentum and powers the jet acceleration and collimation. The jet originates from the inner disk region where equipartition between thermal and magnetic forces is achieved. A hollow, superfast magnetosonic shell of dense material is the natural outcome of the inward advection of a primordial field.

  2. The role of interactions in triggering bars, spiral arms and AGN in disk galaxies

    NASA Astrophysics Data System (ADS)

    Nair, Preethi; Ellison, Sara L.; Patton, David R.

    2016-01-01

    The role of secular structures like bars, rings and spiral arms in triggering star formation and AGN activity in disk galaxies are not well understood. In addition, the mechanisms which create and destroy these structures are not well characterized. Mergers are considered to be one of the main mechanisms which can trigger bars in massive disk galaxies. Using a sample of ~8000 close pair galaxies at 0.02 < z < 0.06 from the Sloan Digital Sky Survey, I will present results illustrating the role of mergers in triggering bars, rings, spiral arms and AGN as a function of close pair separation and merger ratios as well as their dependence on morphology and other physical properties of the galaxies. Time permitting, I will show how resolved IFU observations from SDSS MaNGA will help to place stronger constraints on the role of these structures in triggering star formation and AGN.

  3. Tracing the incidence of X-ray AGN and their distribution of accretion rates across the galaxy population

    NASA Astrophysics Data System (ADS)

    Aird, James; Coil, Alison; Georgakakis, Antonis; Nandra, Kirpal

    2016-08-01

    X-ray selection provides a powerful method of identifying AGN across a variety of host galaxies and with a wide range of accretion rates. However, careful consideration of the underlying selection biases are vital to reveal the true underlying distribution of accretion rates and determine how the incidence of AGN is related to the properties of the galaxies that host them. I will present new measurements of the distribution of specific accretion rates (scaled relative to the total host galaxy mass, roughly tracing the Eddington ratio) within both star-forming and quiescent galaxy populations. We combine near-infrared selected samples of galaxies from the CANDELS/3D-HST and UltraVISTA surveys with deep Chandra X-ray data and use an advanced Bayesian technique to constrain the underlying distribution of specific accretion rates as a function of stellar mass and redshift. Our results reveal a broad distribution of accretion rates (reflecting long-term variability in the level of AGN fuelling) in both galaxy types. The probability of a star-forming galaxy hosting an AGN (above a fixed specific accretion rate) has a strong stellar mass dependence - revealing an intrinsically higher incidence of AGN in massive star-forming galaxies - and undergoes a stellar-mass-dependent evolution with redshift. The probability of a quiescent galaxy hosting an AGN is generally lower but does not depend on stellar mass and evolves differently with redshift. These results provide vital insights into the relationship between the growth of black hole and the physical properties of their host galaxies.

  4. Convective overstability in radially stratified accretion disks under thermal relaxation

    SciTech Connect

    Klahr, Hubert; Hubbard, Alexander

    2014-06-10

    This paper expands the stability criterion for radially stratified, vertically unstratified accretion disks incorporating thermal relaxation. We find a linear amplification of epicyclic oscillations in these disks that depends on the effective cooling time, i.e., an overstability. The growth rates of the overstability vanish for both extreme cases, e.g., infinite cooling time and instantaneous cooling, i.e., the adiabatic and fully isothermal cases. However, for thermal relaxation times τ on the order of the orbital frequency, τΩ ∼ 1, modes grow at a rate proportional to the square of the Brunt-Väisälä frequency. The overstability is based on epicyclic motions, with the thermal relaxation causing gas to heat while radially displaced inward and cool while radially displaced outward. This causes the gas to have a lower density when moving outward compared to when it moves inward, so it feels the outward-directed pressure force more strongly on that leg of the journey. We suggest the term 'convective overstability' for the phenomenon which has already been studied numerically in the nonlinear regime in the context of amplifying vortices in disks under the name 'subcritical baroclinic instability'. The aim of the present paper is to make clear that vortex formation in three-dimensional disks is not necessarily subcritical, i.e., does not need a finite perturbation, nor is it baroclinic in the sense of geophysical fluid dynamics, which requires on vertical shear. We find that convective overstability is a linear instability that will operate under a wide range of physical conditions for circumstellar disks.

  5. Convective Overstability in Radially Stratified Accretion Disks under Thermal Relaxation

    NASA Astrophysics Data System (ADS)

    Klahr, Hubert; Hubbard, Alexander

    2014-06-01

    This paper expands the stability criterion for radially stratified, vertically unstratified accretion disks incorporating thermal relaxation. We find a linear amplification of epicyclic oscillations in these disks that depends on the effective cooling time, i.e., an overstability. The growth rates of the overstability vanish for both extreme cases, e.g., infinite cooling time and instantaneous cooling, i.e., the adiabatic and fully isothermal cases. However, for thermal relaxation times τ on the order of the orbital frequency, τΩ ~ 1, modes grow at a rate proportional to the square of the Brunt-Väisälä frequency. The overstability is based on epicyclic motions, with the thermal relaxation causing gas to heat while radially displaced inward and cool while radially displaced outward. This causes the gas to have a lower density when moving outward compared to when it moves inward, so it feels the outward-directed pressure force more strongly on that leg of the journey. We suggest the term "convective overstability" for the phenomenon which has already been studied numerically in the nonlinear regime in the context of amplifying vortices in disks under the name "subcritical baroclinic instability." The aim of the present paper is to make clear that vortex formation in three-dimensional disks is not necessarily subcritical, i.e., does not need a finite perturbation, nor is it baroclinic in the sense of geophysical fluid dynamics, which requires on vertical shear. We find that convective overstability is a linear instability that will operate under a wide range of physical conditions for circumstellar disks.

  6. Magnetohydrodynamic simulations of global accretion disks with vertical magnetic fields

    SciTech Connect

    Suzuki, Takeru K.; Inutsuka, Shu-ichiro

    2014-04-01

    We report results of three-dimensional magnetohydrodynamical (MHD) simulations of global accretion disks threaded with weak vertical magnetic fields. We perform the simulations in the spherical coordinates with different temperature profiles and accordingly different rotation profiles. In the cases with a spatially constant temperature, because the rotation frequency is vertically constant in the equilibrium condition, general properties of the turbulence excited by magnetorotational instability are quantitatively similar to those obtained in local shearing box simulations. On the other hand, in the cases with a radially variable temperature profile, the vertical differential rotation, which is inevitable in the equilibrium condition, winds up the magnetic field lines in addition to the usual radial differential rotation. As a result, the coherent wound magnetic fields contribute to the Maxwell stress in the surface regions. We obtain nondimensional density and velocity fluctuations ∼0.1-0.2 at the midplane. The azimuthal power spectra of the magnetic fields show shallower slopes, ∼m {sup 0} – m {sup –1}, than those of velocity and density. The Poynting flux associated with the MHD turbulence drives intermittent and structured disk winds as well as sound-like waves toward the midplane. The mass accretion mainly occurs near the surfaces, and the gas near the midplane slowly moves outward in the time domain of the present simulations. The vertical magnetic fields are also dragged inward in the surface regions, while they stochastically move outward and inward around the midplane. We also discuss an observational implication of induced spiral structure in the simulated turbulent disks.

  7. Global Properties of Fully Convective Accretion Disks from Local Simulations

    NASA Astrophysics Data System (ADS)

    Bodo, G.; Cattaneo, F.; Mignone, A.; Ponzo, F.; Rossi, P.

    2015-08-01

    We present an approach to deriving global properties of accretion disks from the knowledge of local solutions derived from numerical simulations based on the shearing box approximation. The approach consists of a two-step procedure. First, a local solution valid for all values of the disk height is constructed by piecing together an interior solution obtained numerically with an analytical exterior radiative solution. The matching is obtained by assuming hydrostatic balance and radiative equilibrium. Although in principle the procedure can be carried out in general, it simplifies considerably when the interior solution is fully convective. In these cases, the construction is analogous to the derivation of the Hayashi tracks for protostars. The second step consists of piecing together the local solutions at different radii to obtain a global solution. Here we use the symmetry of the solutions with respect to the defining dimensionless numbers—in a way similar to the use of homology relations in stellar structure theory—to obtain the scaling properties of the various disk quantities with radius.

  8. ON THE ROLE OF THE ACCRETION DISK IN BLACK HOLE DISK-JET CONNECTIONS

    SciTech Connect

    Miller, J. M.; Reis, R. C.; Pooley, G. G.; Fabian, A. C.; Cackett, E. M.; Nowak, M. A.; Pottschmidt, K.; Wilms, J.

    2012-09-20

    Models of jet production in black hole systems suggest that the properties of the accretion disk-such as its mass accretion rate, inner radius, and emergent magnetic field-should drive and modulate the production of relativistic jets. Stellar-mass black holes in the 'low/hard' state are an excellent laboratory in which to study disk-jet connections, but few coordinated observations are made using spectrometers that can incisively probe the inner disk. We report on a series of 20 Suzaku observations of Cygnus X-1 made in the jet-producing low/hard state. Contemporaneous radio monitoring was done using the Arcminute MicroKelvin Array radio telescope. Two important and simple results are obtained: (1) the jet (as traced by radio flux) does not appear to be modulated by changes in the inner radius of the accretion disk and (2) the jet is sensitive to disk properties, including its flux, temperature, and ionization. Some more complex results may reveal aspects of a coupled disk-corona-jet system. A positive correlation between the reflected X-ray flux and radio flux may represent specific support for a plasma ejection model of the corona, wherein the base of a jet produces hard X-ray emission. Within the framework of the plasma ejection model, the spectra suggest a jet base with v/c {approx_equal} 0.3 or the escape velocity for a vertical height of z {approx_equal} 20 GM/c {sup 2} above the black hole. The detailed results of X-ray disk continuum and reflection modeling also suggest a height of z {approx_equal} 20 GM/c {sup 2} for hard X-ray production above a black hole, with a spin in the range 0.6 {<=} a {<=} 0.99. This height agrees with X-ray time lags recently found in Cygnus X-1. The overall picture that emerges from this study is broadly consistent with some jet-focused models for black hole spectral energy distributions in which a relativistic plasma is accelerated at z = 10-100 GM/c {sup 2}. We discuss these results in the context of disk-jet connections

  9. Imprint of accretion disk-induced migration on gravitational waves from extreme mass ratio inspirals.

    PubMed

    Yunes, Nicolás; Kocsis, Bence; Loeb, Abraham; Haiman, Zoltán

    2011-10-21

    We study the effects of a thin gaseous accretion disk on the inspiral of a stellar-mass black hole into a supermassive black hole. We construct a phenomenological angular momentum transport equation that reproduces known disk effects. Disk torques modify the gravitational wave phase evolution to detectable levels with LISA for reasonable disk parameters. The Fourier transform of disk-modified waveforms acquires a correction with a different frequency trend than post-Newtonian vacuum terms. Such inspirals could be used to detect accretion disks with LISA and to probe their physical parameters. PMID:22107500

  10. Toy model for a two-dimensional accretion disk dominated by Poynting flux

    SciTech Connect

    Lee, Hyun Kyu

    2001-08-15

    We discuss the effect of the Poynting flux on a magnetically dominated thin accretion disk, which is simplified to a two-dimensional disk on the equatorial plane. It is shown in the relativistic formulation that the Poynting flux caused by a rotating magnetic field with Keplerian angular velocity can balance the energy and angular momentum conservation of a stationary accretion flow.

  11. Aperiodic optical variability of intermediate polars - cataclysmic variables with truncated accretion disks

    NASA Astrophysics Data System (ADS)

    Revnivtsev, M.; Burenin, R.; Bikmaev, I.; Kniazev, A.; Buckley, D. A. H.; Pretorius, M. L.; Khamitov, I.; Ak, T.; Eker, Z.; Melnikov, S.; Crawford, S.; Pavlinsky, M.

    2010-04-01

    We study the power spectra of the variability of seven intermediate polars containing magnetized asynchronous accreting white dwarfs, XSS J00564+4548, IGR J00234+6141, DO Dra, V1223 Sgr, IGR J15094-6649, IGR J16500-3307 and IGR J17195-4100, in the optical band and demonstrate that their variability can be well described by a model based on fluctuations propagating in a truncated accretion disk. The power spectra have breaks at Fourier frequencies, which we associate with the Keplerian frequency of the disk at the boundary of the white dwarfs' magnetospheres. We propose that the properties of the optical power spectra can be used to deduce the geometry of the inner parts of the accretion disk, in particular: 1) truncation radii of the magnetically disrupted accretion disks in intermediate polars, 2) the truncation radii of the accretion disk in quiescent states of dwarf novae.

  12. Spectrally resolved eclipse maps of the accretion disk in UX Ursae Majoris

    NASA Technical Reports Server (NTRS)

    Rutten, Rene G. M.; Dhillon, V. S.; Horne, Keith; Kuulkers, E.; Van Paradijs, J.

    1993-01-01

    An effort is made to observationally constrain accretion disks on the basis of light curves from the eclipsing cataclysmic variable UX Ursae Majoris, reconstructing the spectral energy distribution across the face of an accretion disk. The spectral resolution obtained suffices to reveal not only the radial dependence of absorption and emission line features within the disk, but also the spectral details of the bright spot that is formed where the accretion stream from the secondary star collides with the disk. The importance of such constraints for theoretical models is noted.

  13. Probing the Accretion Disk and Central Engine Structure of the NGC 4258 with Suzaku and XMM-Newton Observations

    NASA Technical Reports Server (NTRS)

    Reynolds, Christopher S.; Nowak, Michael A.; Markoff, Sera; Tueller, Jack; Wilms, Joern; Young, Andrew

    2009-01-01

    We present an X-ray study of the low-luminosity active galactic nucleus (AGN) in NGC 4258 using data from Suzaku, XMM-Newton, and the Swift/Burst Alert Telescope survey. We find that signatures of X-ray reprocessing by cold gas are very weak in the spectrum of this Seyfert-2 galaxy; a weak, narrow fluorescent K(alpha) emission line of cod iron is robustly detected in both the Suzaku and XMM-Newton spectra but at a level much below that of most other Seyfert-2 galaxies. We conclude that the circumnuclear environment of this AGN is very "clean" and lacks the Compton-thick obscuring torus of unified Seyfert schemes. From the narrowness of the iron line, together with evidence of line flux variability between the Suzaku and XMM-Newton observations, we constrain the line emitting region to be between 3 x 10(exp 3)r(sub g) and 4 x 10(exp 4)r(sub g), from the black hole. We show that the observed properties of the iron line can be explained if the line originates from the surface layers of a warped accretion disk. In particular, we present explicit calculations of the expected iron line from a disk warped by Lens-Thirring precession from a misaligned central black hole. Finally, the Suzaku data reveal clear evidence of large amplitude 2-10 keV variability on timescales of 50 ksec and smaller amplitude flares on timescales as short as 5-10 ksec. If associated with accretion disk processes, such rapid variability requires an origin in the innermost regions of the disk (r approx. equals 10(r(sub g) or less). Analysis of the difference spectrum between a high- and low-flux states suggests that the variable component of the X-ray emission is steeper and more absorbed than the average AGN emission, suggesting that the primary X-ray source and absorbing screen have a spatial structure on comparable scales. We note the remarkable similarity between the circumnuclear environment of NGC 4258 and another well studied low-luminosity AGN, M81*.

  14. Effects of Accretion Disks on Spins and Eccentricities of Binaries, and Implications for Gravitational Waves

    NASA Technical Reports Server (NTRS)

    Baker, John

    2012-01-01

    Effects of accretion disks on spins and eccentricities of binaries, and implications for gravitational waves. John Baker Space-based gravitational wave observations will allow exquisitely precise measurements of massive black hole binary properties. Through several recently suggested processes, these properties may depend on interactions with accretion disks through the merger process. I will discuss ways that accretion may influence those binary properties which may be probed by gravitational-wave observations.

  15. Evidence for accretion disks in highly polarized quasars

    NASA Technical Reports Server (NTRS)

    Smith, Paul S.; Elston, Richard; Berriman, Graham; Allen, Richard G.; Balonek, Thomas J.

    1988-01-01

    The results of a search for thermal components in 11 highly polarized quasars (HPQs) using UVBRI polarimetry and photometry are reported. The 2000-2500 A luminosities of the thermal components are calculated and the estimated luminosities of the broad-line region (BLR) are given in the same wavelength for comparison. The observed optical continua are modeled as a combination of polarized synchrotron emission, unpolarized emission from the BLR, and an unpolarized flat spectral component that may be optically thick thermal emission from an accretion disk. Evidence for thermal emission components is found in three HPQs: PKS 0420-014, B2 1156+295, and 3C 454.3, with marginal evidence in another two, PKS 1510-089 and PKS 2345-167.

  16. Constraints on jets and accretion disks in low luminosity radio galaxies

    NASA Astrophysics Data System (ADS)

    Tilak, Avanti

    Multiwavelength data has greatly advanced our understanding of AGN and their environments. I study a sample of 21 nearby low luminosity radio galaxies, selected on the basis of distance, radio luminosity, size and Hubble type from the Uppsala Galactic Catalog. I present 1 new and 12 archival Chandra datasets. X-ray imaging of all sources shows nuclear X-ray emission from the AGN. 11 out of these 13 sources also show diffuse emission associated with the host galaxy. I discovered three new X-ray jets. In conjunction with the six known X-ray jets, nine out of the thirteen galaxies exhibit X-ray jet emission. Thus X-ray jets are fairly ubiquitous in low luminosity radio galaxies. The nuclear X-ray spectra are best described by a combination of hot gas and powerlaw models. The nuclear flux and luminosity densities seem well-correlated with their optical and radio counterparts. The correlation is stronger if the contribution from the powerlaw component to the X-ray emission is isolated. Thus the powerlaw component may be the true description of emission from the central region (either from the accretion disk or parsec-scale jet). I investigate the nature of core emission by fitting synchrotron models as well as by comparing disk models with broad-band spectral energy distributions of the AGN. In general, the core emission seems consistent with a synchrotron-jet model. I present VLBA imaging and polarization data for 10 sources in our sample. All 10 sources show corejet morphology, consistent with relativistic jet velocities on parsec scales. The jets are expected to decelerate on scales similar to the extent of the X-ray jets. Assuming synchrotron emission, it is possible that some fraction of the bulk kinetic energy of the jets is being channeled for reacceleration of electrons within the jet, necessary to produce the observed X-ray emission. In the VLBA data, 5 out of 10 sources show polarization. The polarization observations are broadly consistent with unification

  17. Empirical links between XRB and AGN accretion using the complete z < 0.4 spectroscopic CSC/SDSS catalog

    SciTech Connect

    Trichas, Markos; Green, Paul J.; Aldcroft, Tom; Sobolewska, Malgosia; Kim, Dong-Woo; Constantin, Anca; Kalfountzou, Eleni; Hyde, Ashley K.; Zhou, Hongyan; Haggard, Daryl; Kelly, Brandon C.

    2013-12-01

    Striking similarities have been seen between accretion signatures of Galactic X-ray binary (XRB) systems and active galactic nuclei (AGNs). XRB spectral states show a V-shaped correlation between X-ray spectral hardness and Eddington ratio as they vary, and some AGN samples reveal a similar trend, implying analogous processes at vastly larger masses and timescales. To further investigate the analogies, we have matched 617 sources from the Chandra Source Catalog to Sloan Digital Sky Survey spectroscopy, and uniformly measured both X-ray and optical spectral characteristics across a broad range of AGN and galaxy types. We provide useful tabulations of X-ray spectral slope for broad- and narrow-line AGNs, star-forming and passive galaxies, and composite systems, also updating relationships between optical (Hα and [O III]) line emission and X-ray luminosity. We further fit broadband spectral energy distributions with a variety of templates to estimate bolometric luminosity. Our results confirm a significant trend in AGNs between X-ray spectral hardness and Eddington ratio expressed in X-ray luminosity, albeit with significant dispersion. The trend is not significant when expressed in the full bolometric or template-estimated AGN luminosity. We also confirm a relationship between the X-ray/optical spectral slope α{sub ox} and Eddington ratio, but it may not follow the trend predicted by analogy with XRB accretion states.

  18. Black hole accretion disks - Electrodynamic coupling of accretion-disk coronae and the partitioning of soft and hard X-ray emission

    NASA Technical Reports Server (NTRS)

    Kuperus, M.; Ionson, J. A.

    1985-01-01

    It is demonstrated that the observed large ratio of hard to soft X-ray emission and the bimodel behavior of black hole accreting X-ray sources such as Cygnus X-1 can be described in terms of a magnetically structured accretion disk corona which is electrodynamically coupled to the disk turbulent motions while the disk is thermodynamically coupled to the corona as described by a feedback parameter delta. The observed ratio of hard to soft X-ray emission is independent of the disk thickness, and weakly dependent of the disk parameter alpha relating the disk viscous stresses to the total pressure. Observed values of the luminosity ratio point towards strong differences of the feedback of the low state compared to the high state, in the sense that low state means small feedback (delta less than 0.2) and high state means strong feedback delta of about 0.5.

  19. The intrinsic quasar luminosity function: Accounting for accretion disk anisotropy

    SciTech Connect

    DiPompeo, M. A.; Myers, A. D.; Brotherton, M. S.; Runnoe, J. C.; Green, R. F.

    2014-05-20

    Quasar luminosity functions are a fundamental probe of the growth and evolution of supermassive black holes. Measuring the intrinsic luminosity function is difficult in practice, due to a multitude of observational and systematic effects. As sample sizes increase and measurement errors drop, characterizing the systematic effects is becoming more important. It is well known that the continuum emission from the accretion disk of quasars is anisotropic—in part due to its disk-like structure—but current luminosity function calculations effectively assume isotropy over the range of unobscured lines of sight. Here, we provide the first steps in characterizing the effect of random quasar orientations and simple models of anisotropy on observed luminosity functions. We find that the effect of orientation is not insignificant and exceeds other potential corrections such as those from gravitational lensing of foreground structures. We argue that current observational constraints may overestimate the intrinsic luminosity function by as much as a factor of ∼2 on the bright end. This has implications for models of quasars and their role in the universe, such as quasars' contribution to cosmological backgrounds.

  20. NUMERICAL SIMULATIONS OF NATURALLY TILTED, RETROGRADELY PRECESSING, NODAL SUPERHUMPING ACCRETION DISKS

    SciTech Connect

    Montgomery, M. M.

    2012-02-15

    Accretion disks around black hole, neutron star, and white dwarf systems are thought to sometimes tilt, retrogradely precess, and produce hump-shaped modulations in light curves that have a period shorter than the orbital period. Although artificially rotating numerically simulated accretion disks out of the orbital plane and around the line of nodes generate these short-period superhumps and retrograde precession of the disk, no numerical code to date has been shown to produce a disk tilt naturally. In this work, we report the first naturally tilted disk in non-magnetic cataclysmic variables using three-dimensional smoothed particle hydrodynamics. Our simulations show that after many hundreds of orbital periods, the disk has tilted on its own and this disk tilt is without the aid of radiation sources or magnetic fields. As the system orbits, the accretion stream strikes the bright spot (which is on the rim of the tilted disk) and flows over and under the disk on different flow paths. These different flow paths suggest the lift force as a source to disk tilt. Our results confirm the disk shape, disk structure, and negative superhump period and support the source to disk tilt, source to retrograde precession, and location associated with X-ray and He II emission from the disk as suggested in previous works. Our results identify the fundamental negative superhump frequency as the indicator of disk tilt around the line of nodes.

  1. 1 Hz FLARING IN THE ACCRETING MILLISECOND PULSAR NGC 6440 X-2: DISK TRAPPING AND ACCRETION CYCLES

    SciTech Connect

    Patruno, Alessandro; D'Angelo, Caroline

    2013-07-10

    The dynamics of the plasma in the inner regions of an accretion disk around accreting millisecond X-ray pulsars (AMXPs) is controlled by the magnetic field of the neutron star. The interaction between an accretion disk and a strong magnetic field is not well understood, particularly at low accretion rates (the so-called propeller regime). This is due in part to the lack of clear observational diagnostics to constrain the physics of the disk-field interaction. Here, we associate the strong {approx}1 Hz modulation seen in the AMXP NGC 6440 X-2 with an instability that arises when the inner edge of the accretion disk is close to the corotation radius (where the stellar rotation rate matches the Keplerian speed in the disk). A similar modulation has previously been observed in another AMXP (SAX J1808.4-3658) and we suggest that the two phenomena are related and that this may be a common phenomenon among other magnetized systems. Detailed comparisons with theoretical models suggest that when the instability is observed, the interaction region between the disk and the field is very narrow-of the order of 1 km. Modeling further suggests that there is a transition region ({approx}1-10 km) around the corotation radius where the disk-field torque changes sign from spin-up to spin-down. This is the first time that a direct observational constraint has been placed on the width of the disk-magnetosphere interaction region, in the frame of the trapped-disk instability model.

  2. Smearing of mass accretion rate variation by viscous processes in accretion disks in compact binary systems

    NASA Astrophysics Data System (ADS)

    Ghosh, A.; Chakrabarti, Sandip K.

    2016-09-01

    Variation of mass supply rate from the companion can be smeared out by viscous processes inside an accretion disk. Hence, by the time the flow reaches the inner edge, the variation in X-rays need not reflect the true variation of the mass supply rate at the outer edge. However, if the viscosity fluctuates around a mean value, one would expect the viscous time scale t_{{visc}} also to spread around a mean value. In high mass X-ray binaries, which are thought to be primarily wind-fed, the size of the viscous Keplerian disk is smaller and thus such a spread could be lower as compared to the low mass X-ray binaries which are primarily fed by Roche lobe overflow. If there is an increasing or decreasing trend in viscosity, the interval between enhanced emission would be modified systematically. In the absence of a detailed knowledge about the variation of mass supply rates at the outer edge, we study ideal circumstances where modulation must take place exactly in orbital time scales, such as when there is an ellipticity in the orbit. We study a few compact binaries using long term All Sky monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) and all sky survey data (15-50 keV) of Swift satellites by different methods to look for such smearing effects and to infer what these results can tell us about the viscous processes inside the respective disks. We employ three different methods to seek imprints of periodicity on the X-ray variation and found that in all the cases, the location of the peak in the power density spectra is consistent with the orbital frequencies. Interestingly, in high mass X-ray binaries the peaks are sharp with high rms values, consistent with a small Keplerian disk in a wind fed system. However, in low mass X-ray binaries with larger Keplerian disk component, the peaks are spreaded out with much lower rms values. X-ray reflections, or superhump phenomena which may also cause such X-ray modulations would not be affected by the size of

  3. Between the Disk and the Star: Boundary Layers in Astrophysical Accretion Disks

    NASA Astrophysics Data System (ADS)

    Rafikov, Roman

    Accretion disks are encountered in a wide variety of astrophysical settings: compact binaries, active galactic nuclei, star and planet formation. Whenever accretion proceeds onto a central object with a surface, a boundary layer forms at the inner edge of the disk. Energy release taking place in this layer is expected to dramatically modify spectra and give rise to variability of accreting objects. To properly interpret observations it is thus imperative to obtain a good working model of the boundary layer phenomenon. However, at the moment reconciliation of observations and theory of the boundary layers in grossly incomplete, and there are long-standing puzzles such as e.g. the missing boundary layer problem in accreting white dwarfs and young stars. The main reason for this unsatisfactory state of affairs is that the current theory is severely hampered by the lack of understanding of the mechanisms by which angular momentum and mass are transported across the boundary layer region. Our group has recently discovered a novel transport mechanism, which naturally emerges under the boundary layer conditions and has proven to be very robust. Using a suite of large-scale hydrodynamic and magnetohydrodynamic simulations we have shown that supersonic shear near the stellar surface is highly conducive to the excitation of a global instability, which relies on launching the acoustic modes in the system. Saturation of this instability results in a regular, non-axisymmetric, global pattern of acoustic waves that persists on very long time scales. These waves dissipate in weak shocks giving rise to global momentum transport, which in turn drives mass accretion. Transport of angular momentum and energy by these waves is intrinsically non-local and cannot be described in the framework of existing local models of effective viscosity. This new transport mechanism is efficient and robust. Its discovery calls for better and urgent understanding of how the boundary layer operate in a

  4. METAL ACCRETION ONTO WHITE DWARFS CAUSED BY POYNTING-ROBERTSON DRAG ON THEIR DEBRIS DISKS

    SciTech Connect

    Rafikov, Roman R.

    2011-05-01

    Recent discoveries of compact (sizes {approx}disks around more than a dozen metal-rich white dwarfs (WDs) suggest that pollution of these stars with metals may be caused by accretion of high-Z material from the disk. But the mechanism responsible for efficient transfer of mass from a particulate disk to the WD atmosphere has not yet been identified. Here we demonstrate that radiation of the WD can effectively drive accretion of matter through the disk toward the sublimation radius (located at several tens of WD radii), where particles evaporate, feeding a disk of metal gas accreting onto the WD. We show that, contrary to some previous claims, Poynting-Robertson (PR) drag on the debris disk is effective at providing metal accretion rate M-dot{sub PR}{approx}10{sup 8} g s{sup -1} and higher, scaling quadratically with WD effective temperature. We compare our results with observations and show that, as expected, no WD hosting a particulate debris disk shows evidence of metal accretion rate below that produced by the PR drag. Existence of WDs accreting metals at rates significantly higher than M-dot{sub PR} suggests that another mechanism in addition to the PR drag drives accretion of high-Z elements in these systems.

  5. Cosmic Evolution of Accretion Power and Fusion Power: AGN and Starbursts at High Redshifts

    NASA Astrophysics Data System (ADS)

    Arnold Malkan, Matthew

    2009-05-01

    Extragalactic astronomers have been working for decades on obtaining robust measures of the luminosities galaxies produce from stars, and from active galactic nuclei. Our ultimate goal is deriving the cosmic evolution of all radiation produced by fusion and by black hole accretion. The combined effects of dust reddening and redshift make it impossible to achieve this with optical observations alone. Fortunately, infrared thermal continuum and forbidden line emission--from warm dust grains and ionized gas, respectively--can now be measured with excellent sensitivity. However, when measuring entire galaxies, these dust and gas emissions are powered by both active galactic nuclei and starbursts, which may be hard to separate spatially. We must use the fact that the patterns of IR energy output from AGN and SBs differ, with AGN making more ionized gas and hotter dust grains. Low-resolution spectroscopy, or even narrow-band filters can sort out the line emission from both processes when they are mixed in the same galaxy. The hope is that these spectroscopic determinations of star formation rate, and mass accretion rate in relatively small samples of bright galaxies will allow a calibration of broadband continuum measures. The dust continuum emission will then be measured in enormous samples of galaxies spanning their full range of masses, metallicities, environments and redshifts. Along the way, we should learn the astrophysical basis of black hole/galaxy "co-evolution." I will summarize some of the first specific infrared steps of this ambitious agenda, taken with IRAS and ISO to 2MASS, Akari and Spitzer and other telescopes. Time permitting, some of the exciting upcoming observational prospects will be advertised.

  6. Brackett γ radiation from the inner gaseous accretion disk, magnetosphere, and disk wind region of Herbig AeBe stars

    NASA Astrophysics Data System (ADS)

    Tambovtseva, L. V.; Grinin, V. P.; Weigelt, G.

    2016-05-01

    Various disk and outflow components such as the magnetosphere, the disk wind, the gaseous accretion disk, and other regions may contribute to the hydrogen line emission of young Herbig AeBe stars. Non-LTE modeling was performed to show the influence of the model parameters of each emitting region on the intensity and shape of the Brγ line profile, to present the spatial brightness distribution of each component, and to compare the contribution of each component to the total line emission. The modeling shows that the disk wind is the dominant contributor to the Brγ line rather than the magnetosphere and inner gaseous accretion disk. The contribution of the disk wind region to the Hα line is also considered.

  7. Toward a New Paradigm for the Unification of Radio Loud AGN and its Connection to Accretion

    NASA Technical Reports Server (NTRS)

    Georganpoulos, Markos; Meyer, Eileen T.; Fossati, Giovanni; Lister, Matthew L.

    2012-01-01

    We recently argued [21J that the collective properties. of radio loud active galactic nuclei point to the existence of two families of sources, one of powerful sources with single velocity jets and one of weaker jets with significant velocity gradients in the radiating plasma. These families also correspond to different accretion modes and therefore different thermal and emission line intrinsic properties: powerful sources have radiatively efficient accretion disks, while in weak sources accretion must be radiatively inefficient. Here, after we briefly review of our recent work, we present the following findings that support our unification scheme: (i) along the broken sequence of aligned objects, the jet kinetic power increases. (ii) in the powerful branch of the sequence of aligned objects the fraction of BLLs decreases with increasing jet power. (iii) for powerful sources, the fraction of BLLs increases for more un-aligned objects, as measured by the core to extended radio emission. Our results are also compatible with the possibility that a given accretion power produces jets of comparable kinetic power.

  8. STRONG FIELD EFFECTS ON EMISSION LINE PROFILES: KERR BLACK HOLES AND WARPED ACCRETION DISKS

    SciTech Connect

    Wang Yan; Li Xiangdong

    2012-01-10

    If an accretion disk around a black hole is illuminated by hard X-rays from non-thermal coronae, fluorescent iron lines will be emitted from the inner region of the accretion disk. The emission line profiles will show a variety of strong field effects, which may be used as a probe of the spin parameter of the black hole and the structure of the accretion disk. In this paper, we generalize the previous relativistic line profile models by including both the black hole spinning effects and the non-axisymmetries of warped accretion disks. Our results show different features from the conventional calculations for either a flat disk around a Kerr black hole or a warped disk around a Schwarzschild black hole by presenting, at the same time, multiple peaks, rather long red tails, and time variations of line profiles with the precession of the disk. We show disk images as seen by a distant observer, which are distorted by the strong gravity. Although we are primarily concerned with the iron K-shell lines in this paper, the calculation is general and is valid for any emission lines produced from a warped accretion disk around a black hole.

  9. LOW-MASS AGNs AND THEIR RELATION TO THE FUNDAMENTAL PLANE OF BLACK HOLE ACCRETION

    SciTech Connect

    Gültekin, Kayhan; King, Ashley L.; Miller, Jon M.; Cackett, Edward M.; Pinkney, Jason

    2014-06-20

    We put active galactic nuclei (AGNs) with low-mass black holes on the fundamental plane of black hole accretion—the plane that relates X-ray emission, radio emission, and mass of an accreting black hole—to test whether or not the relation is universal for both stellar-mass and supermassive black holes. We use new Chandra X-ray and Very Large Array radio observations of a sample of black holes with masses less than 10{sup 6.3} M {sub ☉}, which have the best leverage for determining whether supermassive black holes and stellar-mass black holes belong on the same plane. Our results suggest that the two different classes of black holes both belong on the same relation. These results allow us to conclude that the fundamental plane is suitable for use in estimating supermassive black hole masses smaller than ∼10{sup 7} M {sub ☉}, in testing for intermediate-mass black holes, and in estimating masses at high accretion rates.

  10. Self-shadowing Effects of Slim Accretion Disks in Active Galactic Nuclei: The Diverse Appearance of the Broad-line Region

    NASA Astrophysics Data System (ADS)

    Wang, Jian-Min; Qiu, Jie; Du, Pu; Ho, Luis C.

    2014-12-01

    Supermassive black holes in active galactic nuclei (AGNs) undergo a wide range of accretion rates, which lead to diversity of appearance. We consider the effects of anisotropic radiation from accretion disks on the broad-line region (BLR) from the Shakura-Sunyaev regime to slim disks with super-Eddington accretion rates. The geometrically thick funnel of the inner region of slim disks produces strong self-shadowing effects that lead to very strong anisotropy of the radiation field. We demonstrate that the degree of anisotropy of the radiation fields grows with increasing accretion rate. As a result of this anisotropy, BLR clouds receive different spectral energy distributions depending on their location relative to the disk, resulting in the diverse observational appearance of the BLR. We show that the self-shadowing of the inner parts of the disk naturally produces two dynamically distinct regions of the BLR, depending on accretion rate. These two regions manifest themselves as kinematically distinct components of the broad Hβ line profile with different line widths and fluxes, which jointly account for the Lorentzian profile generally observed in narrow-line Seyfert 1 galaxies. In the time domain, these two components are expected to reverberate with different time lags with respect to the varying ionizing continuum, depending on the accretion rate and the viewing angle of the observer. The diverse appearance of the BLR due to the anisotropic ionizing energy source can be tested by reverberation mapping of Hβ and other broad emission lines (e.g., Fe II), providing a new tool to diagnose the structure and dynamics of the BLR. Other observational consequences of our model are also explored.

  11. Self-shadowing effects of slim accretion disks in active galactic nuclei: the diverse appearance of the broad-line region

    SciTech Connect

    Wang, Jian-Min; Qiu, Jie; Du, Pu; Ho, Luis C.

    2014-12-10

    Supermassive black holes in active galactic nuclei (AGNs) undergo a wide range of accretion rates, which lead to diversity of appearance. We consider the effects of anisotropic radiation from accretion disks on the broad-line region (BLR) from the Shakura-Sunyaev regime to slim disks with super-Eddington accretion rates. The geometrically thick funnel of the inner region of slim disks produces strong self-shadowing effects that lead to very strong anisotropy of the radiation field. We demonstrate that the degree of anisotropy of the radiation fields grows with increasing accretion rate. As a result of this anisotropy, BLR clouds receive different spectral energy distributions depending on their location relative to the disk, resulting in the diverse observational appearance of the BLR. We show that the self-shadowing of the inner parts of the disk naturally produces two dynamically distinct regions of the BLR, depending on accretion rate. These two regions manifest themselves as kinematically distinct components of the broad Hβ line profile with different line widths and fluxes, which jointly account for the Lorentzian profile generally observed in narrow-line Seyfert 1 galaxies. In the time domain, these two components are expected to reverberate with different time lags with respect to the varying ionizing continuum, depending on the accretion rate and the viewing angle of the observer. The diverse appearance of the BLR due to the anisotropic ionizing energy source can be tested by reverberation mapping of Hβ and other broad emission lines (e.g., Fe II), providing a new tool to diagnose the structure and dynamics of the BLR. Other observational consequences of our model are also explored.

  12. NEUTRINO SPECTRA FROM ACCRETION DISKS: NEUTRINO GENERAL RELATIVISTIC EFFECTS AND THE CONSEQUENCES FOR NUCLEOSYNTHESIS

    SciTech Connect

    Caballero, O. L.; McLaughlin, G. C.; Surman, R. E-mail: olcaball@ncsu.edu E-mail: surmanr@union.edu

    2012-02-01

    Black hole (BH) accretion disks have been proposed as good candidates for a range of interesting nucleosynthesis, including the r-process. The presence of the BH influences the neutrino fluxes and affects the nucleosynthesis resulting from the interaction of the emitted neutrinos and hot outflowing material ejected from the disk. We study the impact of general relativistic effects on the neutrinos emitted from BH accretion disks. We present abundances obtained by considering null geodesics and energy shifts for two different disk models. We find that both the bending of the neutrino trajectories and the energy shifts have important consequences for the nucleosynthetic outcome.

  13. Workshop on Physics of Accretion Disks Around Compact and Young Stars

    NASA Technical Reports Server (NTRS)

    Liang, E (Editor); Stepinski, T. F. (Editor)

    1995-01-01

    The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster presentations, and a list of workshop participants. Accretion disks are believed to surround many stars. Some of these disks form around compact stars, such as white dwarfs, neutron stars, or black holes that are members of binary systems and reveal themselves as a power source, especially in the x-ray and gamma regions of the spectrum. On the other hand, protostellar disks are believed to be accretion disks associated with young, pre-main-sequence stars and manifest themselves mostly in infrared and radio observations. These disks are considered to be a natural outcome of the star formation process. The focus of this workshop included theory and observations relevant to accretion disks around compact objects and newly forming stars, with the primary purpose of bringing the two communities together for intellectual cross-fertilization. The nature of the workshop was exploratory, to see how much interaction is possible between distinct communities and to better realize the local potential in this subject. A critical workshop activity was identification and documentation of key issues that are of mutual interest to both communities.

  14. Alignment of magnetized accretion disks and relativistic jets with spinning black holes.

    PubMed

    McKinney, Jonathan C; Tchekhovskoy, Alexander; Blandford, Roger D

    2013-01-01

    Accreting black holes (BHs) produce intense radiation and powerful relativistic jets, which are affected by the BH's spin magnitude and direction. Although thin disks might align with the BH spin axis via the Bardeen-Petterson effect, this does not apply to jet systems with thick disks. We used fully three-dimensional general relativistic magnetohydrodynamical simulations to study accreting BHs with various spin vectors and disk thicknesses and with magnetic flux reaching saturation. Our simulations reveal a "magneto-spin alignment" mechanism that causes magnetized disks and jets to align with the BH spin near BHs and to reorient with the outer disk farther away. This mechanism has implications for the evolution of BH mass and spin, BH feedback on host galaxies, and resolved BH images for the accreting BHs in SgrA* and M87.

  15. The structure and appearance of winds from supercritical accretion disks. I - Numerical models

    NASA Technical Reports Server (NTRS)

    Meier, D. L.

    1979-01-01

    Equations for the structure and appearance of supercritical accretion disks and the radiation-driven winds which emanate from them are derived and solved by a steady-state hydrodynamic computer code with a relaxation technique used in stellar structure problems. The present model takes into account the mass of the accreting star, the total accretion rate, a generalization of the disk alpha parameter which accounts for heating by processes in addition to viscosity, and the ratio of the total luminosity to the Eddington luminosity. Solutions indicate that for accretion onto a hard-surfaced star, steady, optically thick winds result for even slightly supercritical accretion, and the object will appear as a supergiant star with a high mass loss rate and a nonblackbody spectrum. Winds from black hole accretion disks are expected to depend on the form of the accretion interior to the critical radius, possibly consisting of no ejection at all, a wind similar to that of a hard-surfaced star, or a column of material ejected from a hole in the accretion disk.

  16. Measuring the Relative Contributions of Viscous Accretion and Photoevaporation to the Dispersal of Protoplanetary Disks

    NASA Astrophysics Data System (ADS)

    Simon, M. N.; Pascucci, I.; Rigliaco, E.; Gorti, U.; Hollenbach, D.

    2014-03-01

    Models of protoplanetary disk evolution suggest that photoevaporation driven by the central star and viscous evolution via gas accretion onto the star are the main mechanisms that drive disk dispersal. Viscous evolution has the ability to smoothly decrease the disk surface density, but photoevaporation can drastically change it by creating gaps in planet-forming regions that widen quickly over time. This quick gas dispersal can stop the migration of giant planets whose location affects the final delivery of volatiles (including water) to terrestrial planets. We selected a sample of twenty protoplanetary disks around T. Tauri stars in the Taurus region spanning all three main disk evolutionary stages, with a range of mass accretion rates. For this sample we have acquired high-resolution optical spectra with Keck/HIRES covering gas lines that trace both accretion and photoevaporation. We will present an analysis of the forbidden OI, SII, and NII lines and provide empirically determined mass loss rates as a function of disk evolutionary stage and mass accretion rate. This will enhance our understanding of the disk stage at which photoevaporation starts to dominate over viscous accretion.

  17. ACCRETION OF GAS ONTO GAP-OPENING PLANETS AND CIRCUMPLANETARY FLOW STRUCTURE IN MAGNETIZED TURBULENT DISKS

    SciTech Connect

    Uribe, A. L.; Klahr, H.; Henning, Th.

    2013-06-01

    We have performed three-dimensional magnetohydrodynamical simulations of stellar accretion disks, using the PLUTO code, and studied the accretion of gas onto a Jupiter-mass planet and the structure of the circumplanetary gas flow after opening a gap in the disk. We compare our results with simulations of laminar, yet viscous disks with different levels of an {alpha}-type viscosity. In all cases, we find that the accretion flow across the surface of the Hill sphere of the planet is not spherically or azimuthally symmetric, and is predominantly restricted to the mid-plane region of the disk. Even in the turbulent case, we find no significant vertical flow of mass into the Hill sphere. The outer parts of the circumplanetary disk are shown to rotate significantly below Keplerian speed, independent of viscosity, while the circumplanetary disk density (therefore the angular momentum) increases with viscosity. For a simulation of a magnetized turbulent disk, where the global averaged alpha stress is {alpha}{sub MHD} = 10{sup -3}, we find the accretion rate onto the planet to be M-dot {approx}2 Multiplication-Sign 10{sup -6}M{sub J} yr{sup -1} for a gap surface density of 12 g cm{sup -2}. This is about a third of the accretion rate obtained in a laminar viscous simulation with equivalent {alpha} parameter.

  18. THE STRUCTURE OF THE ACCRETION DISK IN THE ACCRETION DISK CORONA X-RAY BINARY 4U 1822-371 AT OPTICAL AND ULTRAVIOLET WAVELENGTHS

    SciTech Connect

    Bayless, Amanda J.; Robinson, Edward L.; Cornell, Mark E.; Hynes, Robert I.; Ashcraft, Teresa A.

    2010-01-20

    The eclipsing low-mass X-ray binary 4U 1822-371 is the prototypical accretion disk corona (ADC) system. We have obtained new time-resolved UV spectroscopy of 4U 1822-371 with the Advanced Camera for Surveys/Solar Blind Channel on the Hubble Space Telescope and new V- and J-band photometry with the 1.3 m SMARTS telescope at Cerro Tololo Inter-American Observatory. We use the new data to construct its UV/optical spectral energy distribution and its orbital light curve in the UV, V, and J bands. We derive an improved ephemeris for the optical eclipses and confirm that the orbital period is changing rapidly, indicating extremely high rates of mass flow in the system, and we show that the accretion disk in the system has a strong wind with projected velocities up to 4000 km s{sup -1}. We show that the disk has a vertically extended, optically thick component at optical wavelengths. This component extends almost to the edge of the disk and has a height equal to approx0.5 of the disk radius. As it has a low brightness temperature, we identify it as the optically thick base of a disk wind, not as the optical counterpart of the ADC. Like previous models of 4U 1822-371, ours needs a tall obscuring wall near the edge of the accretion disk, but we interpret the wall as a layer of cooler material at the base of the disk wind, not as a tall, luminous disk rim.

  19. Does an Average White Dwarf Have Enough Mass to Prevent Accretion Disk Tilt?

    NASA Astrophysics Data System (ADS)

    Montgomery, M. M.

    2010-11-01

    In a recent publication, we introduce the lift force as a common source to accretion disk tilt that is likely relevant to accretion disk systems. Lift is generated by slightly different supersonic gas stream speeds flowing over and under the disk at the bright spot. In this conference proceeding, we focus on whether the average white dwarf has enough mass to prevent a disk tilt in non-magnetic Cataclysmic Variables (CVs) with accretion disks. Assuming a white dwarf mass of 0.6 Msolar and a disk mass of 10-11 Msolar, we vary the secondary mass to establish theoretical minimum mass transfer rates needed to induce and maintain a disk tilt of four degrees around the line of nodes. For mass ratios in the range (0.13<=q = M2M-1<=0.45), we confirm that the secondary mass does not contribute significantly to disk tilt. We also confirm that the average white dwarf does not have enough mass to prevent a disk tilt. We find that disk tilt may be likely in low mass transfer rate systems such as CV SU UMa's.

  20. Lunar volatile depletion due to incomplete accretion within an impact-generated disk

    NASA Astrophysics Data System (ADS)

    Canup, Robin M.; Visscher, Channon; Salmon, Julien; Fegley, Bruce, Jr.

    2015-12-01

    The Moon may have formed from an Earth-orbiting disk of vapour and melt produced by a giant impact. The mantles of the Moon and Earth have similar compositions. However, it is unclear why lunar samples are more depleted in volatile elements than terrestrial mantle rocks, given that an evaporative escape mechanism seems inconsistent with expected disk conditions. Dynamical models suggest that the Moon initially accreted from the outermost disk, but later acquired up to 60% of its mass from melt originating from the inner disk. Here we combine dynamical, thermal and chemical models to show that volatile depletion in the Moon can be explained by preferential accretion of volatile-rich melt in the inner disk to the Earth, rather than to the growing Moon. Melt in the inner disk is initially hot and volatile poor, but volatiles condense as the disk cools. In our simulations, the delivery of inner disk melt to the Moon effectively ceases when gravitational interactions cause the Moon’s orbit to expand away from the disk, and this termination of lunar accretion occurs before condensation of potassium and more volatile elements. Thus, the portion of the Moon derived from the inner disk is expected to be volatile depleted. We suggest that this mechanism may explain part or all of the Moon’s volatile depletion, depending on the degree of mixing within the lunar interior.

  1. MODERATE-LUMINOSITY GROWING BLACK HOLES FROM 1.25 < z < 2.7: VARIED ACCRETION IN DISK-DOMINATED HOSTS

    SciTech Connect

    Simmons, B. D.; Glikman, E.; Urry, C. M.; Schawinski, K.; Cardamone, C.

    2012-12-10

    We compute black hole masses and bolometric luminosities for 57 active galactic nuclei (AGNs) in the redshift range 1.25 {<=} z {<=} 2.67, selected from the GOODS-South deep multi-wavelength survey field via their X-ray emission. We determine host galaxy morphological parameters by separating the galaxies from their central point sources in deep Hubble Space Telescope images, and host stellar masses and colors by multi-wavelength spectral energy distribution fitting. Of GOODS AGNs at these redshifts, 90% have detected rest-frame optical nuclear point sources; bolometric luminosities range from 2 Multiplication-Sign 10{sup 43} to 2 Multiplication-Sign 10{sup 46} erg s{sup -1}. The black holes are growing at a range of accretion rates, with {approx}> 50% of the sample having L/L{sub Edd} < 0.1. Of the host galaxies, 70% have stellar masses M{sub *} > 10{sup 10} M{sub Sun }, with a range of colors suggesting a complex star formation history. We find no evolution of AGN bolometric luminosity within the sample, and no correlation between AGN bolometric luminosity and host stellar mass, color, or morphology. Fully half the sample of host galaxies are disk-dominated, with another 25% having strong disk components. Fewer than 15% of the systems appear to be at some stage of a major merger. These moderate-luminosity AGN hosts are therefore inconsistent with a dynamical history dominated by mergers strong enough to destroy disks, indicating that minor mergers or secular processes dominate the coevolution of galaxies and their central black holes at z {approx} 2.

  2. Self-collimated electromagnetic jets from magnetized accretion disks - The even-symmetry case

    NASA Technical Reports Server (NTRS)

    Wang, J. C. L.; Sulkanen, M. E.; Lovelace, R. V. E.

    1990-01-01

    This paper extends the previous treatment (Lovelace et al., 1987) of the origin of self-collimated EM jets to the case of even field symmetry, where the magnetic flux function Psi(r, z) is an even function of z. A viscous resistive accretion disk is assumed to surround a black hole with a force-free plasma outside of the disk. Inside the disk, the induction equation is solved for Psi(r, z) and the toroidal magnetic field. Outside the disk, previous results are used to study the formation of self-collimated EM jets. In contrast with the odd-symmetry case, for even symmetry the toroidal magnetic field acts to vertically compress the disk; a comparatively large toroidal magnetic field can exist inside the disk; and an appreciable fraction (possibly all) of the available accretion power can go into the jets.

  3. Black hole accretion disks - Coronal stabilization of the Lightman-Eardley instability

    NASA Technical Reports Server (NTRS)

    Ionson, J. A.; Kuperus, M.

    1984-01-01

    Physical processes by which the presence of a corona around a black hole can raise the threshold of onset of the Lightman-Eardley (L-E, 1976) instability are explored analytically. The L-E model predicts that an optically thick disk becomes unstable when the disk radiation pressure exceeds the disk gas pressure. The model has important implications for the validity of either the coronal disk or two-temperature disk models for accretion zones around black holes. It is shown that a corona can dissipate accreting gravitational energy through radiative cooling. Specific ratios of hard/soft X-rays are quantified for stable and unstable conditions. X-ray spectra from Cyg X-1 are cited as residing below the instability threshold value and thus are supportive of the coronal disk model.

  4. NUSTAR and SUZAKU X-ray spectroscopy of NGC 4151: Evidence for reflection from the inner accretion disk

    SciTech Connect

    Keck, M. L.; Brenneman, L. W.; Ballantyne, D. R.; Bauer, F.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Dauser, T.; Elvis, M.; Fabian, A. C.; Fuerst, F.; García, J.; Grefenstette, B. W.; Hailey, C. J.; Harrison, F. A.; Madejski, G.; Marinucci, A.; Matt, G.; Reynolds, C. S.; Stern, D.; Walton, D. J.; Zoghbi, A.

    2015-06-15

    We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spin $a\\gt 0.9$ accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. As a result, we discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features.

  5. Keplerian Circumbinary Disk and Accretion Streams around the Protostellar Binary System L1551 NE

    NASA Astrophysics Data System (ADS)

    Takakuwa, S.; Saito, M.; Lim, J.; Saigo, K.; Hanawa, T.; Matsumoto, T.

    2013-10-01

    We show our recent observational results of L1551 NE, an archetypal binary protostellar system, in the 0.9-mm dust continuum emission and the C18O (J=3-2) emission with the SubMillimeter Array (SMA). The SMA results show firm evidence for a Keplerian circumbinary disk, circumstellar disks, and an inner clearing in the circumbinary disk, in L1551 NE. We demonstrate that future observations of L1551 NE with Atacama Large Millimeter and submillimeter Array (ALMA) have the potential to unveil the theoretically-predicted “accretion streams” that channel material from the circumbinary disk to the individual circumstellar disks.

  6. An Extensive Numerical Survey of the Correlation Between Outflow Dynamics and Accretion Disk Magnetization

    NASA Astrophysics Data System (ADS)

    Stepanovs, Deniss; Fendt, Christian

    2016-07-01

    We investigate the accretion-ejection process of jets from magnetized accretion disks. We apply a novel approach to the jet-launching problem in order to obtain correlations between the physical properties of the jet and the underlying disk. We extend and confirm the previous works of Tzeferacos et al. and Murphy et al. by scanning a large parameter range for the disk magnetization, {μ }{{D}}={10}-3.5...{10}-0.7. We disentangle the disk magnetization at the foot point of the outflow as the main parameter that governs the properties of the outflow. We show how the four jet integrals known from steady-state MHD are correlated to the disk magnetization at the jet foot point. This agrees with the usual findings of the steady-state theory, however, here we obtain these correlations from time-dependent simulations that include the dynamical evolution of the disk in the treatment. In particular, we obtain robust correlations between the local disk magnetization and (i) the outflow velocity, (ii) the jet mass loading, (iii) the jet angular momentum, and (iv) the local mass accretion rate. Essentially, we find that strongly magnetized disks launch more energetic and faster jets and, due to a larger Alfvén lever arm, these jets extract more angular momentum from the underlying disk. These kinds of disk-jet systems have, however, a smaller mass loading parameter and a lower mass ejection-accretion ratio. The jets are launched at the disk surface where the magnetization is μ (r,z)≃ 0.1. The magnetization rapidly increases vertically providing the energy reservoir for subsequent jet acceleration. We find indications of a critical disk magnetization {μ }{{D}}≃ 0.01 that separates the regimes of magneto-centrifugally driven and magnetic pressure-driven jets.

  7. CONSTRAINTS ON COMPTON-THICK WINDS FROM BLACK HOLE ACCRETION DISKS: CAN WE SEE THE INNER DISK?

    SciTech Connect

    Reynolds, Christopher S.

    2012-11-01

    Strong evidence is emerging that winds can be driven from the central regions of accretion disks in both active galactic nuclei and Galactic black hole binaries. Direct evidence for highly ionized, Compton-thin inner-disk winds comes from observations of blueshifted (v {approx} 0.05-0.1c) iron-K X-ray absorption lines. However, it has been suggested that the inner regions of black hole accretion disks can also drive Compton-thick winds-such winds would enshroud the inner disk, preventing us from seeing direct signatures of the accretion disk (i.e., the photospheric thermal emission, or the Doppler/gravitationally broadened iron K{alpha} line). Here, we show that, provided the source is sub-Eddington, the well-established wind-driving mechanisms fail to launch a Compton-thick wind from the inner disk. For the accelerated region of the wind to be Compton-thick, the momentum carried in the wind must exceed the available photon momentum by a factor of at least 2/{lambda}, where {lambda} is the Eddington ratio of the source, ruling out radiative acceleration unless the source is very close to the Eddington limit. Compton-thick winds also carry large mass fluxes, and a consideration of the connections between the wind and the disk shows this to be incompatible with magneto-centrifugal driving. Finally, thermal driving of the wind is ruled out on the basis of the large Compton radii that typify black hole systems. In the absence of some new acceleration mechanisms, we conclude that the inner regions of sub-Eddington accretion disks around black holes are indeed naked.

  8. Long-term Evolution of Protostellar and Protoplanetary Disks. II. Layered Accretion with Infall

    NASA Astrophysics Data System (ADS)

    Zhu, Zhaohuan; Hartmann, Lee; Gammie, Charles

    2010-04-01

    We use one-dimensional two-zone time-dependent accretion disk models to study the long-term evolution of protostellar disks subject to mass addition from the collapse of a rotating cloud core. Our model consists of a constant surface density magnetically coupled active layer, with transport and dissipation in inactive regions only via gravitational instability. We start our simulations after a central protostar has formed, containing ~10% of the mass of the protostellar cloud. Subsequent evolution depends on the angular momentum of the accreting envelope. We find that disk accretion matches the infall rate early in the disk evolution because much of the inner disk is hot enough to couple to the magnetic field. Later infall reaches the disk beyond ~10 AU, and the disk undergoes outbursts of accretion in FU Ori-like events as described by Zhu et al. If the initial cloud core is moderately rotating, most of the central star's mass is built up by these outburst events. Our results suggest that the protostellar "luminosity problem" is eased by accretion during these FU Ori-like outbursts. After infall stops, the disk enters the T Tauri phase. An outer, viscously evolving disk has a structure that is in reasonable agreement with recent submillimeter studies and its surface density evolves from Σ vprop R -1 to R -1.5. An inner, massive belt of material—the "dead zone"—would not have been observed yet but should be seen in future high angular resolution observations by EVLA and ALMA. This high surface density belt is a generic consequence of low angular momentum transport efficiency at radii where the disk is magnetically decoupled, and would strongly affect planet formation and migration.

  9. CIRCUMVENTING THE RADIATION PRESSURE BARRIER IN THE FORMATION OF MASSIVE STARS VIA DISK ACCRETION

    SciTech Connect

    Kuiper, Rolf; Klahr, Hubert; Beuther, Henrik; Henning, Thomas

    2010-10-20

    We present radiation hydrodynamic simulations of the collapse of massive pre-stellar cores. We treat frequency-dependent radiative feedback from stellar evolution and accretion luminosity at a numerical resolution down to 1.27 AU. In the 2D approximation of axially symmetric simulations, for the first time it is possible to simulate the whole accretion phase (up to the end of the accretion disk epoch) for a forming massive star and to perform a broad scan of the parameter space. Our simulation series evidently shows the necessity to incorporate the dust sublimation front to preserve the high shielding property of massive accretion disks. While confirming the upper mass limit of spherically symmetric accretion, our disk accretion models show a persistent high anisotropy of the corresponding thermal radiation field. This yields the growth of the highest-mass stars ever formed in multi-dimensional radiation hydrodynamic simulations, far beyond the upper mass limit of spherical accretion. Non-axially symmetric effects are not necessary to sustain accretion. The radiation pressure launches a stable bipolar outflow, which grows in angle with time, as presumed from observations. For an initial mass of the pre-stellar host core of 60, 120, 240, and 480 M{sub sun} the masses of the final stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2 M{sub sun}, respectively.

  10. A New Paradigm for Gamma Ray Bursts: Long Term Accretion Rate Modulation by an External Accretion Disk

    NASA Technical Reports Server (NTRS)

    Cannizzo, John; Gehrels, Neil

    2009-01-01

    We present a new way of looking at the very long term evolution of GRBs in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence the efficacy of the process that extracts rotational energy from the black hole and inner accretion disk. The pre-Swift paradigm of achromatic, shallow-to-steep "breaks" in the long term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an "external disk" whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power law decay to the GRB light curves. In this model, the different canonical power law decay segments in the GRB identified by Zhang et al. and Nousek et al. represent different physical states of the accretion disk. We identify a physical disk state with each power law segment.

  11. A NEW PARADIGM FOR GAMMA-RAY BURSTS: LONG-TERM ACCRETION RATE MODULATION BY AN EXTERNAL ACCRETION DISK

    SciTech Connect

    Cannizzo, J. K. E-mail: gehrels@milkyway.gsfc.nasa.gov

    2009-08-01

    We present a new way of looking at the very long-term evolution of gamma-ray bursts (GRBs) in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence the efficacy of the process that extracts rotational energy from the black hole and inner accretion disk. The pre-Swift paradigm of achromatic, shallow-to-steep 'breaks' in the long-term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an 'external disk' whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power-law decay to the GRB light curves. In this model, the different canonical power-law decay segments in the GRB identified by Zhang et al. and Nousek et al. represent different physical states of the accretion disk. We identify a physical disk state with each power-law segment.

  12. Formation of Phobos and Deimos in an extended circum-Martian accretion disk

    NASA Astrophysics Data System (ADS)

    Rosenblatt, Pascal; Charnoz, Sébastien; Dunseath-Terao, Mariko; Dunseath, Kevin; Trinh, Antony

    2015-11-01

    Recent exploration of Phobos by Mars Express has renewed the interest in the origin of the Martian moons, which remains an open issue. New studies have recently focused on accretion of debris in a circum-Martian disk. Craddock (Icarus, 2011) has proposed a scenario of formation of such a disk (about 10-4 to 10-3 % the mass of Mars) from a giant collision occurred early in Mars' history. Rosenblatt and Charnoz (Icarus, 2012) have shown that moonlets can be formed, with Phobos and Deimos masses, in a compact disk close to the planet. Nevertheless, the orbit of these moonlets rapidly recedes back to Mars and the moonlet system vanishes in much less than 1 Gy.Citron at al. (Icarus, 2015) have modeled the formation of a disk of debris resulting from a giant collision but with a larger mass (0.1 % of Mars' mass) than in Craddock (2011). They have shown that most of the disk mass is concentrated close to the planet but a small part can extend beyond the synchronous limit (6 Mars' radii).We study here the accretion of debris in an extended disk, focusing in the edge of this disk where surface density of material is the lowest. However, we take into account the gravitational influence of the more massive bodies formed in the denser part of the disk. Our results highlight the driven mechanisms of accretion in a circum-Martian disk of debris.

  13. Magnetic field strength at the innermost circular orbit in accretion disk of supermassive black hole in active galactic nuclei: comparison with the equipartition value

    NASA Astrophysics Data System (ADS)

    Piotrovich, M. Y.; Buliga, S. D.; Gnedin, Y. N.; Natsvlishvili, T. M.; Silant'ev, N. A.

    2015-06-01

    In this paper we present the results of the determination of the magnetic field strength at the innermost stable circular orbit (ISCO) of an active galactic nuclei (AGN) derived from the polarimetric data for radiation emerging from broad line regions (BLR). These results are obtained by the radiative transfer method that takes into account the effect of Faraday rotation depolarization. The observed polarization degree allows to estimate the value of the magnetic field in the BLR and then to derive the ISCO magnetic field strength using the standard accretion disk model (Shakura and Sunyaev in Astron. Astrophys. 24:337, 1973). We used the polarimetric data obtained by Smith et al. (Mon. Not. R. Astron. Soc. 335:773, 2002) to calculate the values of relativistic jet kinetic power of AGN from the derived values of the magnetic field strength at the ISCO radius.

  14. THE BROAD LINE REGION IN NGC 4051: AN INFLOW ILLUMINATED BY A 10{sup 5} K ACCRETION DISK

    SciTech Connect

    Devereux, Nick; Heaton, Emily E-mail: heatone@erau.edu

    2013-08-20

    Adopting a spherically symmetric steady-state ballistic inflow as the kinematic model for the gas distribution responsible for producing the H{alpha} emission line, and a central black hole (BH) mass of 1.7 Multiplication-Sign 10{sup 6} M{sub Sun} determined from prior reverberation mapping, leads to the following dimensions for the size of the broad line region (BLR) in NGC 4051; an inner radius {approx}3 lt-day and a lower limit to the outer radius {approx}475 lt-day. Thus, the previously determined reverberation size for the BLR marks just the inner radius of a much larger volume of ionized gas. The number of ionizing photons required to sustain the H{alpha} emission line luminosity exceeds the number observed to be available from the central active galactic nucleus (AGN) by a factor of 3-4. Such a large ionizing deficit can be reconciled if the BLR is ionized by a 10{sup 5} K accretion disk that is hidden from direct view by the high opacity of intervening H gas. A new definition is introduced for the ionization parameter that acknowledges the fact that H opacity significantly attenuates the flux of ionizing photons in the large, partially ionized nebula surrounding the AGN. Collectively, the results have important implications for BH masses estimated using reverberation radii and the structure of the BLR inferred from velocity-delay maps.

  15. Inner Accretion Disk Regions of Black Hole X-ray Binaries

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg

    2015-01-01

    The innermost regions of accretion disks in black hole X-ray binaries dominate the observed X-ray emission, which is the main diagnostic that one uses to gain insights into the physics of black holes and accretion. The standard spectrum predicted from a geometrically thin, optically thick disk experiences non-trivial modification due to conspiring physical effects operating within the vertical disk structure such as Comptonization, free-free emission/absorption, bound-free opacities, and energy dissipation by magnetic processes. The complicated interplay of these effects cause the seed accretion disk spectrum to become hardened and it is this hardened emergent spectrum that we observe. To zeroth order, this hardening can be described by a phenomenological parameter called the spectral hardening factor.In practice, the adopted degree of spectral hardening is confined to lie within a rather restrictive range. I will discuss the following consequences of relaxing this criterion, while still requiring the spectral hardening factor to take on physically plausible values. Examining multiple state transitions of the black hole X-ray binary GX 339-4 with archival data from the Rossi X-ray Timing Explorer, I will show that appealing to a spectral hardening factor that varies during state transitions provides a viable alternative to a truncated disk model for the evolution of the inner accretion disk. Having demonstrated that moderate degrees of accretion disk spectral hardening cannot be ruled out by observations, I will explore this possibility from a theoretical standpoint. Extending previous work on radiative transfer modeling coupled to the vertical disk structure, I present the impacts on the emergent accretion disk spectrum caused by disk inclination and by allowing accretion power to be dissipated in the corona. Using magnetohydrodynamic simulations of a localized patch of the accretion disk (i.e., shearing box) performed with the Athena code, I will present the

  16. Integrated accretion disk angular momentum removal and astrophysical jet acceleration mechanism

    NASA Astrophysics Data System (ADS)

    Bellan, Paul

    2015-11-01

    A model has been developed for how accretion disks discard angular momentum while powering astrophysical jets. The model depends on the extremely weak ionization of disks. This causes disk ions to be collisionally locked to adjacent disk neutrals so a clump of disk ions and neutrals has an effective cyclotron frequency αωci where α is the fractional ionization. When αωci is approximately twice the Kepler orbital frequency, conservation of canonical momentum shows that the clump spirals radially inwards producing a radially inward disk electric current as electrons cannot move radially in the disk. Upon reaching the jet radius, this current then flows axially away from the disk plane along the jet, producing a toroidal magnetic field that drives the jet. Electrons remain frozen to poloidal flux surfaces everywhere and electron motion on flux surfaces in the ideal MHD region outside the disk completes the current path. Angular momentum absorbed from accreting material in the disk by magnetic counter-torque -JrBz is transported by the electric circuit and ejected at near infinite radius in the disk plane. This is like an electric generator absorbing angular momentum and wired to a distant electric motor that emits angular momentum. Supported by USDOE/NSF Partnership in Plasma Science.

  17. Ultra-fast outflows (aka UFOs) from AGNs and QSOs

    NASA Astrophysics Data System (ADS)

    Cappi, M.; Tombesi, F.; Giustini, M.

    During the last decade, strong observational evidence has been accumulated for the existence of massive, high velocity winds/outflows (aka Ultra Fast Outflows, UFOs) in nearby AGNs and in more distant quasars. Here we briefly review some of the most recent developments in this field and discuss the relevance of UFOs for both understanding the physics of accretion disk winds in AGNs, and for quantifying the global amount of AGN feedback on the surrounding medium.

  18. Binary black hole accretion from a circumbinary disk: Gas dynamics inside the central cavity

    SciTech Connect

    Farris, Brian D.; Duffell, Paul; MacFadyen, Andrew I.; Haiman, Zoltan

    2014-03-10

    We present the results of two-dimensional (2D) hydrodynamical simulations of circumbinary disk accretion using the finite-volume code DISCO. This code solves the 2D viscous Navier-Stokes equations on a high-resolution moving mesh which shears with the fluid flow, greatly reducing advection errors in comparison with a fixed grid. We perform a series of simulations for binary mass ratios in the range 0.026 ≤ q ≤ 1.0, each lasting longer than a viscous time so that we reach a quasi-steady accretion state. In each case, we find that gas is efficiently stripped from the inner edge of the circumbinary disk and enters the cavity along accretion streams, which feed persistent 'mini disks' surrounding each black hole. We find that for q ≳ 0.1, the binary excites eccentricity in the inner region of the circumbinary disk, creating an overdense lump which gives rise to enhanced periodicity in the accretion rate. The dependence of the periodicity on mass ratio may provide a method for observationally inferring mass ratios from measurements of the accretion rate. We also find that for all mass ratios studied, the magnitude of the accretion onto the secondary is sufficient to drive the binary toward larger mass ratio. This suggests a mechanism for biasing mass-ratio distributions toward equal mass.

  19. Can neutron stars have auroras ? : electromagnetic coupling process between neutron star and magnetized accretion disk

    NASA Astrophysics Data System (ADS)

    Kimura, T.; Iwakiri, W. B.; Enoto, T.; Wada, T.; Tao, C.

    2015-12-01

    In the binary neutron star system, angular momentum transfer from accretion disk to a star is essential process for spin-up/down of stars. The angular momentum transfer has been well formulated for the accretion disk strongly magnetized by the neutron star [e.g., Ghosh and Lamb, 1978, 1979a, b]. However, the electromagnetic (EM) coupling between the neutron star and accretion disk has not been self-consistently solved in the previous studies although the magnetic field lines from the star are strongly tied with the accretion disk. In this study, we applied the planet-magnetosphere coupling process established for Jupiter [Hill, 1979] to the binary neutron star system. Angular momentum distribution is solved based on the torque balance between the neutron star's surface and accretion disk coupled by the magnetic field tensions. We found the EM coupling can transfer significantly larger fraction of the angular momentum from the magnetized accretion disk to the star than the unmagnetized case. The resultant spin-up rate is estimated to ~10^-14 [sec/sec] for the nominal binary system parameters, which is comparable with or larger than the other common spin-down/up processes: e.g., the magnetic dipole radiation spin-down. The Joule heating energy dissipated in the EM coupling is estimated to be up to ~10^36 [erg/sec] for the nominal binary system parameters. The release is comparable to that of gravitation energy directly caused by the matters accreting onto the neutron star. This suggests the EM coupling at the neutron star can accompany the observable radiation as auroras with a similar manner to those at the rotating planetary magnetospheres like Jupiter, Saturn, and other gas giants.

  20. Dynamics of accretion disks in a constant curvature f(R)-gravity

    NASA Astrophysics Data System (ADS)

    Alipour, N.; Khesali, A. R.; Nozari, K.

    2016-07-01

    So far the basic physical properties of matter forming a thin accretion disc in the static and spherically symmetric space-time metric of the vacuum f(R) modified gravity models (Pun et al. in Phys. Rev. D 78:024043, 2008) and building radiative models of thin accretion disks for both Schwarzschild and Kerr black holes in f(R) gravity (Perez et al. in Astron. Astrophys. 551:4, 2013) were addressed properly. Also von Zeipel surfaces and convective instabilities in f(R)-Schwarzschild(Kerr) background have been investigated recently (Alipour et al. in Mon. Not. R. Astron. Soc. 454:1992, 2015). In this streamline, here we study the effects of radial and angular pressure gradients on thick accretion disks in Schwarzschild geometries in a constant curvature f(R) modified gravity. Since thick accretion disks have high accretion rate, we study configuration and structure of thick disks by focusing on the effect of pressure gradient on formation of the disks. We clarify our study by assuming two types of equation of state: polytropic and Clapeyron equation of states.

  1. RADIATION PRESSURE-SUPPORTED ACCRETION DISKS: VERTICAL STRUCTURE, ENERGY ADVECTION, AND CONVECTIVE STABILITY

    SciTech Connect

    Gu Weimin

    2012-07-10

    By taking into account the local energy balance per unit volume between the viscous heating and the advective cooling plus the radiative cooling, we investigate the vertical structure of radiation pressure-supported accretion disks in spherical coordinates. Our solutions show that the photosphere of the disk is close to the polar axis and therefore the disk seems to be extremely thick. However, the density profile implies that most of the accreted matter exists in a moderate range around the equatorial plane. We show that the well-known polytropic relation between the pressure and the density is unsuitable for describing the vertical structure of radiation pressure-supported disks. More importantly, we find that the energy advection is significant even for slightly sub-Eddington accretion disks. We argue that the non-negligible advection may help us understand why the standard thin disk model is likely to be inaccurate above {approx}0.3 Eddington luminosity, which was found by some works on black hole spin measurement. Furthermore, the solutions satisfy the Solberg-Hoiland conditions, which indicate the disk to be convectively stable. In addition, we discuss the possible link between our disk model and ultraluminous X-ray sources.

  2. Testing the Propagating Fluctuations Model with a Long, Global Accretion Disk Simulation

    NASA Astrophysics Data System (ADS)

    Hogg, J. Drew; Reynolds, Christopher S.

    2016-07-01

    The broadband variability of many accreting systems displays characteristic structures; log-normal flux distributions, root-mean square (rms)-flux relations, and long inter-band lags. These characteristics are usually interpreted as inward propagating fluctuations of the mass accretion rate in an accretion disk driven by stochasticity of the angular momentum transport mechanism. We present the first analysis of propagating fluctuations in a long-duration, high-resolution, global three-dimensional magnetohydrodynamic (MHD) simulation of a geometrically thin (h/r ≈ 0.1) accretion disk around a black hole. While the dynamical-timescale turbulent fluctuations in the Maxwell stresses are too rapid to drive radially coherent fluctuations in the accretion rate, we find that the low-frequency quasi-periodic dynamo action introduces low-frequency fluctuations in the Maxwell stresses, which then drive the propagating fluctuations. Examining both the mass accretion rate and emission proxies, we recover log-normality, linear rms-flux relations, and radial coherence that would produce inter-band lags. Hence, we successfully relate and connect the phenomenology of propagating fluctuations to modern MHD accretion disk theory.

  3. Studies of Thermally Unstable Accretion Disks Around Black Holes with Adaptive Pseudospectral Domain Decomposition Method. II. Limit-cycle Behavior in Accretion Disks around Kerr Black Holes

    NASA Astrophysics Data System (ADS)

    Xue, Li; Sądowski, Aleksander; Abramowicz, Marek A.; Lu, Ju-Fu

    2011-07-01

    For the first time ever, we derive equations governing the time evolution of fully relativistic slim accretion disks in the Kerr metric and numerically construct their detailed non-stationary models. We discuss applications of these general results to a possible limit-cycle behavior of thermally unstable disks. Our equations and numerical method are applicable in a wide class of possible viscosity prescriptions, but in this paper we use a diffusive form of the "standard alpha prescription" that assumes that the viscous torque is proportional to the total pressure. In this particular case, we find that the parameters that dominate the limit-cycle properties are the mass-supply rate and the value of the alpha-viscosity parameter. Although the duration of the cycle (or the outburst) does not exhibit any clear dependence on the black hole spin, the maximal outburst luminosity (in the Eddington units) is positively correlated with the spin value. We suggest a simple method for a rough estimate of the black hole spin based on the maximal luminosity and the ratio of outburst to cycle durations. We also discuss a temperature-luminosity relation for the Kerr black hole accretion disk limit cycle. Based on these results, we discuss the limit-cycle behavior observed in microquasar GRS 1915+105. We also extend this study to several non-standard viscosity prescriptions, including a "delayed heating" prescription recently addressed by the MHD simulations of accretion disks.

  4. STUDIES OF THERMALLY UNSTABLE ACCRETION DISKS AROUND BLACK HOLES WITH ADAPTIVE PSEUDOSPECTRAL DOMAIN DECOMPOSITION METHOD. II. LIMIT-CYCLE BEHAVIOR IN ACCRETION DISKS AROUND KERR BLACK HOLES

    SciTech Connect

    Xue Li; Lu Jufu; Sadowski, Aleksander; Abramowicz, Marek A. E-mail: lujf@xmu.edu.cn

    2011-07-01

    For the first time ever, we derive equations governing the time evolution of fully relativistic slim accretion disks in the Kerr metric and numerically construct their detailed non-stationary models. We discuss applications of these general results to a possible limit-cycle behavior of thermally unstable disks. Our equations and numerical method are applicable in a wide class of possible viscosity prescriptions, but in this paper we use a diffusive form of the 'standard alpha prescription' that assumes that the viscous torque is proportional to the total pressure. In this particular case, we find that the parameters that dominate the limit-cycle properties are the mass-supply rate and the value of the alpha-viscosity parameter. Although the duration of the cycle (or the outburst) does not exhibit any clear dependence on the black hole spin, the maximal outburst luminosity (in the Eddington units) is positively correlated with the spin value. We suggest a simple method for a rough estimate of the black hole spin based on the maximal luminosity and the ratio of outburst to cycle durations. We also discuss a temperature-luminosity relation for the Kerr black hole accretion disk limit cycle. Based on these results, we discuss the limit-cycle behavior observed in microquasar GRS 1915+105. We also extend this study to several non-standard viscosity prescriptions, including a 'delayed heating' prescription recently addressed by the MHD simulations of accretion disks.

  5. Complex organic molecules along the accretion flow in isolated and externally irradiated protoplanetary disks.

    PubMed

    Walsh, Catherine; Herbst, Eric; Nomura, Hideko; Millar, T J; Weaver, Susanna Widicus

    2014-01-01

    The birth environment of the Sun will have influenced the physical and chemical structure of the pre-solar nebula, including the attainable chemical complexity reached in the disk, important for prebiotic chemistry. The formation and distribution of complex organic molecules (COMs) in a disk around a T Tauri star is investigated for two scenarios: (i) an isolated disk, and (ii) a disk irradiated externally by a nearby massive star. The chemistry is calculated along the accretion flow from the outer disk inwards using a comprehensive network which includes gas-phase reactions, gas-grain interactions, and thermal grain-surface chemistry. Two simulations are performed, one beginning with complex ices and one with simple ices only. For the isolated disk, COMs are transported without major chemical alteration into the inner disk where they thermally desorb into the gas reaching an abundance representative of the initial assumed ice abundance. For simple ices, COMs can efficiently form on grain surfaces under the conditions in the outer disk. Gas-phase COMs are released into the molecular layer via photodesorption. For the irradiated disk, complex ices are also transported inwards; however, they undergo thermal processing caused by the warmer conditions in the irradiated disk which tends to reduce their abundance along the accretion flow. For simple ices, grain-surface chemistry cannot efficiently synthesise COMs in the outer disk because the necessary grain-surface radicals, which tend to be particularly volatile, are not sufficiently abundant on the grain surfaces. Gas-phase COMs are formed in the inner region of the irradiated disk via gas-phase chemistry induced by the desorption of strongly bound molecules such as methanol; hence, the abundances are not representative of the initial molecular abundances injected into the outer disk. These results suggest that the composition of comets formed in isolated disks may differ from those formed in externally irradiated

  6. Relativistic Lines and Reflection from the Inner Accretion Disks Around Neutron Stars

    NASA Astrophysics Data System (ADS)

    Cackett, Edward M.; Miller, Jon M.; Ballantyne, David R.; Barret, Didier; Bhattacharyya, Sudip; Boutelier, Martin; Miller, M. Coleman; Strohmayer, Tod E.; Wijnands, Rudy

    2010-09-01

    A number of neutron star low-mass X-ray binaries (LMXBs) have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. We present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star LMXBs, all of which display broad Fe K emission lines. Of the 10 sources, 4 are Z sources, 4 are atolls, and 2 are accreting millisecond X-ray pulsars (also atolls). The Fe K lines are fit well by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6-15 GM/c 2) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of luminosities. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8 to 20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer illuminates a geometrically thin disk.

  7. RELATIVISTIC LINES AND REFLECTION FROM THE INNER ACCRETION DISKS AROUND NEUTRON STARS

    SciTech Connect

    Cackett, Edward M.; Miller, Jon M.; Ballantyne, David R.; Barret, Didier; Boutelier, Martin; Miller, M. Coleman; Strohmayer, Tod E.

    2010-09-01

    A number of neutron star low-mass X-ray binaries (LMXBs) have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. We present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star LMXBs, all of which display broad Fe K emission lines. Of the 10 sources, 4 are Z sources, 4 are atolls, and 2 are accreting millisecond X-ray pulsars (also atolls). The Fe K lines are fit well by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6-15 GM/c {sup 2}) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of luminosities. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8 to 20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer illuminates a geometrically thin disk.

  8. ACCRETION RATE AND THE PHYSICAL NATURE OF UNOBSCURED ACTIVE GALAXIES

    SciTech Connect

    Trump, Jonathan R.; Impey, Christopher D.; Gabor, Jared M.; Diamond-Stanic, Aleksandar M.; Kelly, Brandon C.; Civano, Francesca; Hao, Heng; Lanzuisi, Giorgio; Merloni, Andrea; Salvato, Mara; Urry, C. Megan; Jahnke, Knud; Nagao, Tohru; Taniguchi, Yoshi; Koekemoer, Anton M.; Liu, Charles; Mainieri, Vincenzo; Scoville, Nick Z.

    2011-05-20

    We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rates by using accurate intrinsic accretion luminosities (L{sub int}) from well-sampled multiwavelength spectral energy distributions from the Cosmic Evolution Survey, and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L{sub int}/L{sub Edd} > 10{sup -2}), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L{sub int}/L{sub Edd} < 10{sup -2}) are unobscured and yet lack a broad-line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L{sub int}/L{sub Edd} < 10{sup -2} narrow-line and lineless AGNs to have ratios of radio-to-optical/UV emission that are 10 times higher than L{sub int}/L{sub Edd} > 10{sup -2} broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L{sub int}/L{sub Edd} < 10{sup -2} AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together, these results suggest that specific accretion rate is an important physical 'axis' of AGN unification, as described by a simple model.

  9. Critical condition for the propeller effect in systems with magnetized neutron stars accreting from geometrically thin accretion disks

    NASA Astrophysics Data System (ADS)

    Ertan, Unal

    2016-07-01

    The inner disk radius around a magnetized neutron star in the spin-down phase is usually assumed to be close to the radius at which the viscous and magnetic stresses are balanced. With different assumptions, this radius is estimated to be very close the Alfven radius. Furthermore, it is commonly assumed that the propeller mechanism can expel the matter from the system when this radius is found to be greater than the co-rotation radius. In the present work, we have shown with simple analytical calculations from the first principles that a steady-state propeller mechanism cannot be established at the radius where the viscous and the magnetic torques are balanced. We have found that a steady-state propeller phase can be built up with an inner disk radius that is at least ~10 - 30 times smaller than the Alfven radius depending on the current mass-flow rate of the disk, the field strength and the rotational period of the source. This result also indicates that the critical accretion rate for the accretion-propeller transition is orders of magnitude smaller than the rate found by equating the Alfven and the co-rotation radii. Our results are consistent with the properties of recently discovered transitional millisecond pulsars which show transitions between the rotational powered radio pulsar and the accretion powered X-ray pulsar states.

  10. Lightman-Eardley instabilities and accretion disk thickening. [for compact astronomical objects

    NASA Technical Reports Server (NTRS)

    Stoeger, W. R.

    1979-01-01

    After reviewing the role of Compton scattering in accretion disks around black holes, it is discussed whether Lightman-Eardley (LE) secular instabilities can trigger and maintain Pringle-Rees (PR) thermal instabilities. The radiative-transfer-equation and equation-of-state criteria for LE stability in alpha-viscosity-law disk models and dynamic viscosity criteria for more general situations is derived. On the basis of these considerations the LE instability is insufficient for inducing PR instabilities and hot thick inner regions important in accretion-disk models of compact hard X-ray sources. The density thinning due to radial velocity gradients in the accretion flow is suggested as a more likely and satisfactory mechanism.

  11. USING MEGAMASER DISKS TO PROBE BLACK HOLE ACCRETION

    SciTech Connect

    Greene, Jenny E.; Sun Ailei; Seth, Anil; Den Brok, Mark; Braatz, James A.; Impellizzeri, C. M. Violette; Lo, K. Y.; Henkel, Christian; Peng, Chien Y.; Kuo, Cheng-Yu

    2013-07-10

    We examine the alignment between H{sub 2}O megamaser disks on subparsec scales with circumnuclear disks and bars on <500 pc scales observed with the Hubble Space Telescope (HST)/WFC3. The HST imaging reveals young stars, indicating the presence of gas. The megamaser disks are not well aligned with the circumnuclear bars or disks as traced by stars in the HST images. We speculate on the implications of the observed misalignments for fueling supermassive black holes in gas-rich spiral galaxies. In contrast, we find a strong preference for the rotation axes of the megamaser disks to align with radio continuum jets observed on {approx}> 50 pc scales, in those galaxies for which radio continuum detections are available. Subarcsecond observations of molecular gas with ALMA will enable a more complete understanding of the interplay between circumnuclear structures.

  12. Vertical dissipation profiles and the photosphere location in thin and slim accretion disks

    NASA Astrophysics Data System (ADS)

    Sadowski, A.; Abramowicz, M. A.; Bursa, M.; Kluźniak, W.; Różańska, A.; Straub, O.

    2009-07-01

    As several authors in the past, we calculate optically thick but geometrically thin (and slim) accretion disk models and perform a ray-tracing of photons in the Kerr geometry to calculate the observed disk continuum spectra. Previously, it was common practice to ray-trace photons assuming that they are emitted from the Kerr geometry equatorial plane, z = 0. We show that the continuum spectra calculated with this assumption differ from these calculated under the assumption that photons are emitted from the actual surface of the disc, z = H(r). This implies that a knowledge of the location of the thin disk effective photosphere is relevant for calculating the continuum emission. In this paper we investigate, in terms of a simple model, a possible influence of the (unknown, and therefore assumed ad hoc) vertical dissipation profiles on the vertical structure of the disk and thus on the location of the effective photosphere, and on the observed continuum spectra. For disks with moderate and high mass accretion rates (dot m > 0.01 dot m_C), we find that the photosphere location in the inner, radiation pressure dominated, disk region (where most of the radiation comes from) does not depend on the dissipation profile and therefore emerging disk spectra are insensitive to the choice of the dissipation function. For lower accretion rates, the photosphere location depends on the assumed vertical dissipation profile down to the disk inner edge, but the dependence is very weak and thus of minor importance. We conclude that the continuum spectra of optically thick accretion disks around black holes should be calculated with ray-tracing from the effective photosphere and that, fortunately, the choice of a particular vertical dissipation profile does not substantially influence the calculated emission.

  13. Evidence for large temperature fluctuations in quasar accretion disks from spectral variability

    SciTech Connect

    Ruan, John J.; Anderson, Scott F.; Agol, Eric; Dexter, Jason

    2014-03-10

    The well-known bluer-when-brighter trend observed in quasar variability is a signature of the complex processes in the accretion disk and can be a probe of the quasar variability mechanism. Using a sample of 604 variable quasars with repeat spectra in the Sloan Digital Sky Survey-I/II (SDSS), we construct difference spectra to investigate the physical causes of this bluer-when-brighter trend. The continuum of our composite difference spectrum is well fit by a power law, with a spectral index in excellent agreement with previous results. We measure the spectral variability relative to the underlying spectra of the quasars, which is independent of any extinction, and compare to model predictions. We show that our SDSS spectral variability results cannot be produced by global accretion rate fluctuations in a thin disk alone. However, we find that a simple model of an inhomogeneous disk with localized temperature fluctuations will produce power-law spectral variability over optical wavelengths. We show that the inhomogeneous disk will provide good fits to our observed spectral variability if the disk has large temperature fluctuations in many independently varying zones, in excellent agreement with independent constraints from quasar microlensing disk sizes, their strong UV spectral continuum, and single-band variability amplitudes. Our results provide an independent constraint on quasar variability models and add to the mounting evidence that quasar accretion disks have large localized temperature fluctuations.

  14. An Extensive Numerical Survey of the Correlation Between Outflow Dynamics and Accretion Disk Magnetization

    NASA Astrophysics Data System (ADS)

    Stepanovs, Deniss; Fendt, Christian

    2016-07-01

    We investigate the accretion–ejection process of jets from magnetized accretion disks. We apply a novel approach to the jet-launching problem in order to obtain correlations between the physical properties of the jet and the underlying disk. We extend and confirm the previous works of Tzeferacos et al. and Murphy et al. by scanning a large parameter range for the disk magnetization, {μ }{{D}}={10}-3.5...{10}-0.7. We disentangle the disk magnetization at the foot point of the outflow as the main parameter that governs the properties of the outflow. We show how the four jet integrals known from steady-state MHD are correlated to the disk magnetization at the jet foot point. This agrees with the usual findings of the steady-state theory, however, here we obtain these correlations from time-dependent simulations that include the dynamical evolution of the disk in the treatment. In particular, we obtain robust correlations between the local disk magnetization and (i) the outflow velocity, (ii) the jet mass loading, (iii) the jet angular momentum, and (iv) the local mass accretion rate. Essentially, we find that strongly magnetized disks launch more energetic and faster jets and, due to a larger Alfvén lever arm, these jets extract more angular momentum from the underlying disk. These kinds of disk–jet systems have, however, a smaller mass loading parameter and a lower mass ejection–accretion ratio. The jets are launched at the disk surface where the magnetization is μ (r,z)≃ 0.1. The magnetization rapidly increases vertically providing the energy reservoir for subsequent jet acceleration. We find indications of a critical disk magnetization {μ }{{D}}≃ 0.01 that separates the regimes of magneto-centrifugally driven and magnetic pressure-driven jets.

  15. Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks

    SciTech Connect

    Fragile, P C; Lindner, C C; Anninos, P; Salmonson, J D

    2008-09-24

    In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing 'plunging streams' and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured 'cubed-sphere' grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.

  16. Thin Disk Accretion in the Magnetically-Arrested State

    NASA Astrophysics Data System (ADS)

    Avara, Mark J.; McKinney, Jonathan; Reynolds, Christopher S.

    2016-01-01

    Shakura-Sunyaev thin disk theory is fundamental to black hole astrophysics. Though applications of the theory are wide-spread and powerful tools for explaining observations, such as Soltan's argument using quasar power, broadened iron line measurements, continuum fitting, and recently reverberation mapping, a significant large-scale magnetic field causes substantial deviations from standard thin disk behavior. We have used fully 3D general relativistic MHD simulations with cooling to explore the thin (H/R~0.1) magnetically arrested disk (MAD) state and quantify these deviations. This work demonstrates that accumulation of large-scale magnetic flux into the MAD state is possible, and then extends prior numerical studies of thicker disks, allowing us to measure how jet power scales with the disk state, providing a natural explanation of phenomena like jet quenching in the high-soft state of X-ray binaries. We have also simulated thin MAD disks with a misaligned black hole spin axis in order to understand further deviations from thin disk theory that may significantly affect observations.

  17. Three-dimensional radiative transfer calculations on an SIMD machine applied to accretion disks

    NASA Technical Reports Server (NTRS)

    Vath, H.

    1994-01-01

    We have developed a tool to solve the radiative transfer equation for a three-dimensional astrophysical object on the SIMD computer MasPar MP-1. With this tool we can rapidly calculate the image of such an object as seen from an arbitrary direction and at an arbitrary wavelength. Such images and spectra can then be used to directly compare observations with the model. This tool can be applied to many different areas in astrophysics, e.g., HI disks of galaxies and polarized radiative transfer of accretion columns onto white dwarfs. Here we use this tool to calculate the image and spectrum of a simple model of an accretion disk.

  18. r-Process Nucleosynthesis in Hot Accretion Disk Flows from Black Hole-Neutron Star Mergers

    SciTech Connect

    Surman, Rebecca; Mclaughlin, Gail C; Ruffert, Maximilian; Janka, Hans-Thomas; Hix, William Raphael

    2008-01-01

    We consider hot accretion disk outflows from black hole-neutron star mergers in the context of the nucleosynthesis they produce. We begin with a three-dimensional numerical model of a black hole-neutron star merger and calculate the neutrino and antineutrino fluxes emitted from the resulting accretion disk. We then follow the element synthesis in material outflowing the disk along parameterized trajectories. We find that at least a weak r-process is produced, and in some cases a main r-process as well. The neutron-rich conditions required for this production of r-process nuclei stem directly from the interactions of the neutrinos emitted by the disk with the free neutrons and protons in the outflow.

  19. r-Process Nucleosynthesis in Hot Accretion Disk Flows from Black Hole - Neutron Star Mergers

    SciTech Connect

    Surman, Rebecca; Mclaughlin, Gail C; Ruffert, Maximilian; Janka, Hans-Thomas; Hix, William Raphael

    2008-01-01

    We consider hot accretion disk outflows from black hole-neutron star mergers in the context of the nucleosynthesis they produce. We begin with a three-dimensional numerical model of a black hole-neutron star merger and calculate the neutrino and antineutrino fluxes emitted from the resulting accretion disk. We then follow the element synthesis in material outflowing the disk along parameterized trajectories. We find that at least a weak r-process is produced, and in some cases a main r-process as well. The neutron-rich conditions required for this production of r-process nuclei stem directly from the interactions of the neutrinos emitted by the disk with the free neutrons and protons in the outflow.

  20. VERTICAL STRUCTURE OF STATIONARY ACCRETION DISKS WITH A LARGE-SCALE MAGNETIC FIELD

    SciTech Connect

    Bisnovatyi-Kogan, G. S.; Lovelace, R. V. E. E-mail: RVL1@cornell.edu

    2012-05-10

    In earlier works we pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the hydrodynamic and/or magnetorotational instabilities are suppressed high in the disk where the magnetic and radiation pressures are larger than the plasma thermal pressure. Here, we calculate the vertical profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity and the fact that the turbulence vanishes at the surface of the disk. Also, here we require that the radial accretion speed be zero at the disk's surface and we assume that the ratio of the turbulent viscosity to the turbulent magnetic diffusivity is of order unity. Thus, at the disk's surface there are three boundary conditions. As a result, for a fixed dimensionless viscosity {alpha}-value, we find that there is a definite relation between the ratio R of the accretion power going into magnetic disk winds to the viscous power dissipation and the midplane plasma-{beta}, which is the ratio of the plasma to magnetic pressure in the disk. For a specific disk model with R of order unity we find that the critical value required for a stationary solution is {beta}{sub c} Almost-Equal-To 2.4r/({alpha}h), where h is the disk's half thickness. For weaker magnetic fields, {beta} > {beta}{sub c}, we argue that the poloidal field will advect outward while for {beta} < {beta}{sub c} it will advect inward. Alternatively, if the disk wind is negligible (R<<1), there are stationary solutions with {beta} >> {beta}{sub c}.

  1. A Brgamma Probe of Disk Accretion in T Tauri Stars and Embedded Young Stellar Objects

    NASA Astrophysics Data System (ADS)

    Muzerolle, James; Hartmann, Lee; Calvet, Nuria

    1998-12-01

    We report on observations of Pabeta and Brgamma for a sample of classical T Tauri stars in Taurus and find a tight correlation between the emission-line luminosities and the accretion luminosity as measured from the hot continuum excess. We use the Brgamma luminosity correlation to calculate accretion luminosities in highly reddened young stars with existing line measurements. The distribution of accretion luminosities is similar in Taurus and Ophiuchus Class II sources. For the deeply embedded Class I objects, the accretion luminosities are in general less than the bolometric luminosities, which implies that the disk accretion rates are significantly lower than the envelope infall rates. We find that the central sources of many Class I objects are quite similar to their Class II counterparts.

  2. Wind-driven Accretion in Protoplanetary Disks. II. Radial Dependence and Global Picture

    NASA Astrophysics Data System (ADS)

    Bai, Xue-Ning

    2013-08-01

    Non-ideal magnetohydrodynamical effects play a crucial role in determining the mechanism and efficiency of angular momentum transport as well as the level of turbulence in protoplanetary disks (PPDs), which are the key to understanding PPD evolution and planet formation. It was shown in our previous work that at 1 AU, the magnetorotational instability (MRI) is completely suppressed when both ohmic resistivity and ambipolar diffusion (AD) are taken into account, resulting in a laminar flow with accretion driven by magnetocentrifugal wind. In this work, we study the radial dependence of the laminar wind solution using local shearing-box simulations. The scaling relation on the angular momentum transport for the laminar wind is obtained, and we find that the wind-driven accretion rate can be approximated as \\dot{M}≈ 0.91× 10-8R_AU1.21(B_p/10 mG)0.93 M\\bigodot yr-1, where Bp is the strength of the large-scale poloidal magnetic field threading the disk. The result is independent of disk surface density. Four criteria are outlined for the existence of the laminar wind solution: (1) ohmic resistivity dominated the midplane region, (2) the AD-dominated disk upper layer, (3) the presence of a (not too weak) net vertical magnetic flux, and (4) sufficiently well-ionized gas beyond the disk surface. All these criteria are likely to be met in the inner region of the disk from ~0.3 AU to about 5-10 AU for typical PPD accretion rates. Beyond this radius, the angular momentum transport is likely to proceed due to a combination of the MRI and disk wind, and eventually completely dominated by the MRI (in the presence of strong AD) in the outer disk. Our simulation results provide key ingredients for a new paradigm on the accretion processes in PPDs.

  3. WIND-DRIVEN ACCRETION IN PROTOPLANETARY DISKS. II. RADIAL DEPENDENCE AND GLOBAL PICTURE

    SciTech Connect

    Bai Xuening

    2013-08-01

    Non-ideal magnetohydrodynamical effects play a crucial role in determining the mechanism and efficiency of angular momentum transport as well as the level of turbulence in protoplanetary disks (PPDs), which are the key to understanding PPD evolution and planet formation. It was shown in our previous work that at 1 AU, the magnetorotational instability (MRI) is completely suppressed when both ohmic resistivity and ambipolar diffusion (AD) are taken into account, resulting in a laminar flow with accretion driven by magnetocentrifugal wind. In this work, we study the radial dependence of the laminar wind solution using local shearing-box simulations. The scaling relation on the angular momentum transport for the laminar wind is obtained, and we find that the wind-driven accretion rate can be approximated as M-dot approx. 0.91 x 10{sup -8}R{sub AU}{sup 1.21}(B{sub p}/10 mG){sup 0.93} M{sub Sun} yr{sup -1}, where B{sub p} is the strength of the large-scale poloidal magnetic field threading the disk. The result is independent of disk surface density. Four criteria are outlined for the existence of the laminar wind solution: (1) ohmic resistivity dominated the midplane region, (2) the AD-dominated disk upper layer, (3) the presence of a (not too weak) net vertical magnetic flux, and (4) sufficiently well-ionized gas beyond the disk surface. All these criteria are likely to be met in the inner region of the disk from {approx}0.3 AU to about 5-10 AU for typical PPD accretion rates. Beyond this radius, the angular momentum transport is likely to proceed due to a combination of the MRI and disk wind, and eventually completely dominated by the MRI (in the presence of strong AD) in the outer disk. Our simulation results provide key ingredients for a new paradigm on the accretion processes in PPDs.

  4. Simulating the Formation of Massive Protostars. I. Radiative Feedback and Accretion Disks

    NASA Astrophysics Data System (ADS)

    Klassen, Mikhail; Pudritz, Ralph E.; Kuiper, Rolf; Peters, Thomas; Banerjee, Robi

    2016-05-01

    We present radiation hydrodynamic simulations of collapsing protostellar cores with initial masses of 30, 100, and 200 M ⊙. We follow their gravitational collapse and the formation of a massive protostar and protostellar accretion disk. We employ a new hybrid radiative feedback method blending raytracing techniques with flux-limited diffusion for a more accurate treatment of the temperature and radiative force. In each case, the disk that forms becomes Toomre-unstable and develops spiral arms. This occurs between 0.35 and 0.55 freefall times and is accompanied by an increase in the accretion rate by a factor of 2–10. Although the disk becomes unstable, no other stars are formed. In the case of our 100 and 200 M ⊙ simulations, the star becomes highly super-Eddington and begins to drive bipolar outflow cavities that expand outwards. These radiatively driven bubbles appear stable, and appear to be channeling gas back onto the protostellar accretion disk. Accretion proceeds strongly through the disk. After 81.4 kyr of evolution, our 30 M ⊙ simulation shows a star with a mass of 5.48 M ⊙ and a disk of mass 3.3 M ⊙, while our 100 M ⊙ simulation forms a 28.8 M ⊙ mass star with a 15.8 M ⊙ disk over the course of 41.6 kyr, and our 200 M ⊙ simulation forms a 43.7 M ⊙ star with an 18 M ⊙ disk in 21.9 kyr. In the absence of magnetic fields or other forms of feedback, the masses of the stars in our simulation do not appear to be limited by their own luminosities.

  5. Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical SatelliteAccretion

    SciTech Connect

    Kazantzidis, Stelios; Bullock, James S.; Zentner, Andrew R.; Kravtsov, Andrey V.; Moustakas, Leonidas A.

    2007-12-03

    We conduct a series of high-resolution, fully self-consistent dissipation less N-body simulations to investigate the cumulative effect of substructure mergers onto thin disk galaxies in the context of the {Lambda}CDM paradigm of structure formation. Our simulation campaign is based on a hybrid approach combining cosmological simulations and controlled numerical experiments. Substructure mass functions, orbital distributions, internal structures, and accretion times are culled directly from cosmological simulations of galaxy-sized cold dark matter (CDM) halos. We demonstrate that accretions of massive subhalos onto the central regions of host halos, where the galactic disk resides, since z {approx} 1 should be common occurrences. In contrast, extremely few satellites in present-day CDM halos are likely to have a significant impact on the disk structure. This is due to the fact that massive subhalos with small orbital pericenters that are most capable of strongly perturbing the disk become either tidally disrupted or suffer substantial mass loss prior to z = 0. One host halo merger history is subsequently used to seed controlled N-body experiments of repeated satellite impacts on an initially-thin Milky Way-type disk galaxy. These simulations track the effects of six dark matter substructures, with initial masses in the range {approx} (0.7-2) x 10{sup 10} M{sub {circle_dot}} ({approx} 20-60% of the disk mass), crossing the disk in the past {approx} 8 Gyr. We show that these accretion events produce several distinctive observational signatures in the stellar disk including: a long-lived, low-surface brightness, ring-like feature in the outskirts; a significant flare; a central bar; and faint filamentary structures that (spuriously) resemble tidal streams in configuration space. The final distribution of disk stars exhibits a complex vertical structure that is well-described by a standard 'thin-thick' disk decomposition, where the 'thick' disk component has emerged

  6. The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared.

    PubMed

    Kishimoto, Makoto; Antonucci, Robert; Blaes, Omer; Lawrence, Andy; Boisson, Catherine; Albrecht, Marcus; Leipski, Christian

    2008-07-24

    Quasars are thought to be powered by supermassive black holes accreting surrounding gas. Central to this picture is a putative accretion disk which is believed to be the source of the majority of the radiative output. It is well known, however, that the most extensively studied disk model-an optically thick disk which is heated locally by the dissipation of gravitational binding energy-is apparently contradicted by observations in a few major respects. In particular, the model predicts a specific blue spectral shape asymptotically from the visible to the near-infrared, but this is not generally seen in the visible wavelength region where the disk spectrum is observable. A crucial difficulty has been that, towards the infrared, the disk spectrum starts to be hidden under strong, hot dust emission from much larger but hitherto unresolved scales, and thus has essentially been impossible to observe. Here we report observations of polarized light interior to the dust-emitting region that enable us to uncover this near-infrared disk spectrum in several quasars. The revealed spectra show that the near-infrared disk spectrum is indeed as blue as predicted. This indicates that, at least for the outer near-infrared-emitting radii, the standard picture of the locally heated disk is approximately correct.

  7. VERTICAL STRUCTURE AND CORONAL POWER OF ACCRETION DISKS POWERED BY MAGNETOROTATIONAL-INSTABILITY TURBULENCE

    SciTech Connect

    Uzdensky, Dmitri A.

    2013-10-01

    In this paper, we consider two outstanding intertwined problems in modern high-energy astrophysics: (1) the vertical-thermal structure of an optically thick accretion disk heated by the dissipation of magnetohydrodynamic turbulence driven by the magnetorotational instability (MRI), and (2) determining the fraction of the accretion power released in the corona above the disk. For simplicity, we consider a gas-pressure-dominated disk and assume a constant opacity. We argue that the local turbulent dissipation rate due to the disruption of the MRI channel flows by secondary parasitic instabilities should be uniform across most of the disk, almost up to the disk photosphere. We then obtain a self-consistent analytical solution for the vertical thermal structure of the disk, governed by the balance between the heating by MRI turbulence and the cooling by radiative diffusion. Next, we argue that the coronal power fraction is determined by the competition between the Parker instability, viewed as a parasitic instability feeding off of MRI channel flows, and other parasitic instabilities. We show that the Parker instability inevitably becomes important near the disk surface, leading to a certain lower limit on the coronal power. While most of the analysis in this paper focuses on the case of a disk threaded by an externally imposed vertical magnetic field, we also discuss the zero net flux case, in which the magnetic field is produced by the MRI dynamo itself, and show that most of our arguments and conclusions should be valid in this case as well.

  8. MIGRATION OF EXTRASOLAR PLANETS: EFFECTS FROM X-WIND ACCRETION DISKS

    SciTech Connect

    Adams, Fred C.; Cai, Mike J.; Lizano, Susana

    2009-09-10

    Magnetic fields are dragged in from the interstellar medium during the gravitational collapse that forms star/disk systems. Consideration of mean field magnetohydrodynamics in these disks shows that magnetic effects produce sub-Keplerian rotation curves and truncate the inner disk. This Letter explores the ramifications of these predicted disk properties for the migration of extrasolar planets. Sub-Keplerian flow in gaseous disks drives a new migration mechanism for embedded planets and modifies the gap-opening processes for larger planets. This sub-Keplerian migration mechanism dominates over Type I migration for sufficiently small planets (m{sub P} {approx}< 1 M {sub +}) and/or close orbits (r {approx}< 1 AU). Although the inclusion of sub-Keplerian torques shortens the total migration time by only a moderate amount, the mass accreted by migrating planetary cores is significantly reduced. Truncation of the inner disk edge (for typical system parameters) naturally explains final planetary orbits with periods P {approx} 4 days. Planets with shorter periods, P {approx} 2 days, can be explained by migration during FU-Orionis outbursts, when the mass accretion rate is high and the disk edge moves inward. Finally, the midplane density is greatly increased at the inner truncation point of the disk (the X-point); this enhancement, in conjunction with continuing flow of gas and solids through the region, supports the in situ formation of giant planets.

  9. Time-dependent X-ray emission from unstable accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Mineshige, Shin; Kim, Soon-Wook; Wheeler, J. Craig

    1990-01-01

    The spectral evolution of accretion disks in X-ray binaries containing black holes is studied, based on the disk instability model. The thermal transition of the outer portions of the disk controls the mass flow rate into the inner portions of the disk, thus modulating the soft X-ray flux which is thought to arise from the inner disk. Calculated soft X-ray spectra are consistent with the observations of the X-ray transient A0620 - 00 and especially ASM 2000 + 25, the soft X-ray spectra of which are well fitted by blackbody radiation with a fixed inner edge of the disk, Rin, and with monotonically decreasing temperature at Rin with time. Since the gas pressure is always dominant over the radiation pressure during the decay in these models, a two-temperature region is difficult to create. Instead, it is suggested that hard X-rays are generated in a hot (kT greater than 10 keV) accretion disk corona above the cool (kT less than 1 keV) disk.

  10. Quasi-static model of collimated jets and radio lobes. I. Accretion disk and jets

    SciTech Connect

    Colgate, Stirling A.; Li, Hui; Fowler, T. Kenneth; Pino, Jesse

    2014-07-10

    This is the first of a series of papers showing that when an efficient dynamo can be maintained by accretion disks around supermassive black holes in active galactic nuclei, it can lead to the formation of a powerful, magnetic helix that could explain both the observed radio jet/lobe structures on very large scales and ultimately the enormous power inferred from the observed ultra-high-energy cosmic rays. In this work, we solve a set of one-dimensional equations similar to the steady-state standard accretion disk model, but now including the large-scale magnetic fields giving rises to jets. We find that the frequently made assumption that large-scale fields are frozen into the disk is fundamentally incorrect, due to the necessity for current and the accreting mass to flow perpendicular to magnetic flux surfaces. A correct treatment greatly simplifies the calculations, yielding fields that leave the disk nearly vertically with magnetic profiles uniquely determined by disk angular momentum conservation. Representative solutions of the magnetic fields in different radial regions of the disk surface are given, and they determine the overall key features in the jet structure and its dissipation, which will be the subjects of later papers.

  11. Circumbinary Ring, Circumstellar Disks, and Accretion in the Binary System UY Aurigae

    NASA Astrophysics Data System (ADS)

    Tang, Ya-Wen; Dutrey, Anne; Guilloteau, Stéphane; Piétu, Vincent; Di Folco, Emmanuel; Beck, Tracy; Ho, Paul T. P.; Boehler, Yann; Gueth, Fréderic; Bary, Jeff; Simon, Michal

    2014-09-01

    Recent exo-planetary surveys reveal that planets can orbit and survive around binary stars. This suggests that some fraction of young binary systems which possess massive circumbinary (CB) disks may be in the midst of planet formation. However, there are very few CB disks detected. We revisit one of the known CB disks, the UY Aurigae system, and probe 13CO 2-1, C18O 2-1, SO 5(6)-4(5) and 12CO 3-2 line emission and the thermal dust continuum. Our new results confirm the existence of the CB disk. In addition, the circumstellar (CS) disks are clearly resolved in dust continuum at 1.4 mm. The spectral indices between the wavelengths of 0.85 mm and 6 cm are found to be surprisingly low, being 1.6 for both CS disks. The deprojected separation of the binary is 1.''26 based on our 1.4 mm continuum data. This is 0.''07 (10 AU) larger than in earlier studies. Combining the fact of the variation of UY Aur B in R band, we propose that the CS disk of an undetected companion UY Aur Bb obscures UY Aur Ba. A very complex kinematical pattern inside the CB disk is observed due to a mixing of Keplerian rotation of the CB disk, the infall and outflow gas. The streaming gas accreting from the CB ring toward the CS disks and possible outflows are also identified and resolved. The SO emission is found to be at the bases of the streaming shocks. Our results suggest that the UY Aur system is undergoing an active accretion phase from the CB disk to the CS disks. The UY Aur B might also be a binary system, making the UY Aur a triple system.

  12. Axisymmetric Two-Dimensional Computation of Magnetic Field Dragging in Accretion Disks

    NASA Technical Reports Server (NTRS)

    Reyes-Ruiz, Mauricio; Stepinski, Tomasz F.

    1996-01-01

    In this paper we model a geometrically thin accretion disk interacting with an externally imposed, uniform, vertical magnetic field. The accretion flow in the disk drags and distorts field lines, amplifying the magnetic field in the process. Inside the disk the radial component of the field is sheared into a toroidal component. The aim of this work is to establish the character of the resultant magnetic field and its dependence on the disk's parameters. We concentrate on alpha-disks driven by turbulent viscosity. Axisymmetric, two-dimensional solutions are obtained without taking into account the back-reaction of the magnetic field on the structure of the disk. The character of the magnetic field depends strongly on the magnitude of the magnetic Prandtl number, P . We present two illustrative examples of viscous disks: a so-called 'standard' steady state model of a disk around a compact star (e.g., cataclysmic variable), and a steady state model of a proto-planetary disk. In both cases, P = 1, P = 10(sup -1), and P = 10(sup -2) scenarios are calculated. Significant bending and magnification of the magnetic field is possible only for disks characterized by P of the order of 10(sup -2). In such a case, the field lines are bent sufficiently to allow the development of a centrifugally driven wind. Inside the disk the field is dominated by its toroidal component. We also investigate the dragging of the magnetic field by a nonviscous protoplanetary disk described by a phenomenological model. This scenario leads to large distortion and magnification of the magnetic field.

  13. The Behavior of Accretion Disks in Low Mass X-ray Binaries: Disk Winds and Alpha Model

    NASA Astrophysics Data System (ADS)

    Bayless, Amanda J.

    2010-01-01

    This dissertation presents research on two low mass X-ray binaries. The eclipsing low-mass X-ray binary 4U 1822-371 is the prototypical accretion disk corona (ADC) system. We have obtained new time-resolved UV spectroscopy with the ACS/SBC on the Hubble Space Telescope and new V- and J-band photometry with the 1.3-m SMARTS telescope at CTIO. We show that the accretion disk in the system has a strong wind with projected velocities up to 4000 km/s as determined from the Doppler width of the C IV emission line. The broad and shallow eclipse indicates that the disk has a vertically-extended, optically-thick component at optical wavelengths. This component extends almost to the edge of the disk and has a height equal to 50% of the disk radius. As it has a low brightness temperature, we identify it as the optically-thick base of the disk wind. V1408 Aql (=4U 1957+115) is a low mass X-ray binary which continues to be a black hole candidate. We have new photometric data of this system from the Otto Struve 2.1-m telescope's high speed CCD photometer at McDonald Observatory. The light curve is largely sinusoidal which we model with two components: a constant light source from the disk and a sinusoidal modulation at the orbital period from the irradiated face of the companion star. This is a radical re-interpretation of the orbital light curve. We do not require a large or asymmetric disk rim to account for the modulation in the light curve. Thus, the orbital inclination is unconstrained in our new model, removing the foundation for any claims of the compact object being a black hole.

  14. Evidence for a correlation between mass accretion rates onto young stars and the mass of their protoplanetary disks

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Rosotti, G.; Testi, L.; Natta, A.; Alcalá, J. M.; Williams, J. P.; Ansdell, M.; Miotello, A.; van der Marel, N.; Tazzari, M.; Carpenter, J.; Guidi, G.; Mathews, G. S.; Oliveira, I.; Prusti, T.; van Dishoeck, E. F.

    2016-06-01

    A relation between the mass accretion rate onto the central young star and the mass of the surrounding protoplanetary disk has long been theoretically predicted and observationally sought. For the first time, we have accurately and homogeneously determined the photospheric parameters, mass accretion rate, and disk mass for an essentially complete sample of young stars with disks in the Lupus clouds. Our work combines the results of surveys conducted with VLT/X-Shooter and ALMA. With this dataset we are able to test a basic prediction of viscous accretion theory, the existence of a linear relation between the mass accretion rate onto the central star and the total disk mass. We find a correlation between the mass accretion rate and the disk dust mass, with a ratio that is roughly consistent with the expected viscous timescale when assuming an interstellar medium gas-to-dust ratio. This confirms that mass accretion rates are related to the properties of the outer disk. We find no correlation between mass accretion rates and the disk mass measured by CO isotopologues emission lines, possibly owing to the small number of measured disk gas masses. This suggests that the mm-sized dust mass better traces the total disk mass and that masses derived from CO may be underestimated, at least in some cases.

  15. The Torquing of Circumnuclear Accretion Disks by Stars and the Evolution of Massive Black Holes

    NASA Astrophysics Data System (ADS)

    Bregman, Michal; Alexander, Tal

    2012-03-01

    An accreting massive black hole (MBH) in a galactic nucleus is surrounded by a dense stellar cluster. We analyze and simulate numerically the evolution of a thin accretion disk due to its internal viscous torques, due to the frame-dragging torques of a spinning MBH (the Bardeen-Petterson effect), and due to the orbit-averaged gravitational torques by the stars (resonant relaxation). We show that the evolution of the MBH mass accretion rate, the MBH spin growth rate, and the covering fraction of the disk relative to the central ionizing continuum source, are all strongly coupled to the stochastic fluctuations of the stellar potential via the warps that the stellar torques excite in the disk. These lead to fluctuations by factors of up to a few in these quantities over a wide range of timescales, with most of the power on timescales >~ (M •/Md )P(Rd ), where M • and Md are the masses of the MBH and disk, and P is the orbital period at the disk's mass-weighted mean radius Rd . The response of the disk is stronger the lighter it is and the more centrally concentrated the stellar cusp. As proof of concept, we simulate the evolution of the low-mass maser disk in NGC 4258 and show that its observed O(10°) warp can be driven by the stellar torques. We also show that the frame dragging of a massive active galactic nucleus disk couples the stochastic stellar torques to the MBH spin and can excite a jitter of a few degrees in its direction relative to that of the disk's outer regions.

  16. THE TORQUING OF CIRCUMNUCLEAR ACCRETION DISKS BY STARS AND THE EVOLUTION OF MASSIVE BLACK HOLES

    SciTech Connect

    Bregman, Michal; Alexander, Tal

    2012-03-20

    An accreting massive black hole (MBH) in a galactic nucleus is surrounded by a dense stellar cluster. We analyze and simulate numerically the evolution of a thin accretion disk due to its internal viscous torques, due to the frame-dragging torques of a spinning MBH (the Bardeen-Petterson effect), and due to the orbit-averaged gravitational torques by the stars (resonant relaxation). We show that the evolution of the MBH mass accretion rate, the MBH spin growth rate, and the covering fraction of the disk relative to the central ionizing continuum source, are all strongly coupled to the stochastic fluctuations of the stellar potential via the warps that the stellar torques excite in the disk. These lead to fluctuations by factors of up to a few in these quantities over a wide range of timescales, with most of the power on timescales {approx}> (M{sub .}/M{sub d} )P(R{sub d} ), where M{sub .} and M{sub d} are the masses of the MBH and disk, and P is the orbital period at the disk's mass-weighted mean radius R{sub d}. The response of the disk is stronger the lighter it is and the more centrally concentrated the stellar cusp. As proof of concept, we simulate the evolution of the low-mass maser disk in NGC 4258 and show that its observed O(10 Degree-Sign ) warp can be driven by the stellar torques. We also show that the frame dragging of a massive active galactic nucleus disk couples the stochastic stellar torques to the MBH spin and can excite a jitter of a few degrees in its direction relative to that of the disk's outer regions.

  17. Circumstellar disks of the most vigorously accreting young stars.

    PubMed

    Liu, Hauyu Baobab; Takami, Michihiro; Kudo, Tomoyuki; Hashimoto, Jun; Dong, Ruobing; Vorobyov, Eduard I; Pyo, Tae-Soo; Fukagawa, Misato; Tamura, Motohide; Henning, Thomas; Dunham, Michael M; Karr, Jennifer L; Kusakabe, Nobuhiko; Tsuribe, Toru

    2016-02-01

    Stars may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. Our high-resolution near-infrared imaging has verified the presence of the key associated features, large-scale arms and arcs surrounding four young stellar objects undergoing luminous outbursts. Our hydrodynamics simulations and radiative transfer models show that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation phase. The effect of those tempestuous episodes of disk evolution on star and planet formation remains to be understood. PMID:26989772

  18. Circumstellar disks of the most vigorously accreting young stars

    PubMed Central

    Liu, Hauyu Baobab; Takami, Michihiro; Kudo, Tomoyuki; Hashimoto, Jun; Dong, Ruobing; Vorobyov, Eduard I.; Pyo, Tae-Soo; Fukagawa, Misato; Tamura, Motohide; Henning, Thomas; Dunham, Michael M.; Karr, Jennifer L.; Kusakabe, Nobuhiko; Tsuribe, Toru

    2016-01-01

    Stars may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. Our high-resolution near-infrared imaging has verified the presence of the key associated features, large-scale arms and arcs surrounding four young stellar objects undergoing luminous outbursts. Our hydrodynamics simulations and radiative transfer models show that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation phase. The effect of those tempestuous episodes of disk evolution on star and planet formation remains to be understood. PMID:26989772

  19. Circumstellar disks of the most vigorously accreting young stars.

    PubMed

    Liu, Hauyu Baobab; Takami, Michihiro; Kudo, Tomoyuki; Hashimoto, Jun; Dong, Ruobing; Vorobyov, Eduard I; Pyo, Tae-Soo; Fukagawa, Misato; Tamura, Motohide; Henning, Thomas; Dunham, Michael M; Karr, Jennifer L; Kusakabe, Nobuhiko; Tsuribe, Toru

    2016-02-01

    Stars may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. Our high-resolution near-infrared imaging has verified the presence of the key associated features, large-scale arms and arcs surrounding four young stellar objects undergoing luminous outbursts. Our hydrodynamics simulations and radiative transfer models show that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation phase. The effect of those tempestuous episodes of disk evolution on star and planet formation remains to be understood.

  20. On the thermal stability of radiation-dominated accretion disks

    SciTech Connect

    Jiang, Yan-Fei; Stone, James M.; Davis, Shane W.

    2013-11-20

    We study the long-term thermal stability of radiation-dominated disks in which the vertical structure is determined self-consistently by the balance of heating due to the dissipation of MHD turbulence driven by magneto-rotational instability (MRI) and cooling due to radiation emitted at the photosphere. The calculations adopt the local shearing box approximation and utilize the recently developed radiation transfer module in the Athena MHD code based on a variable Eddington tensor rather than an assumed local closure. After saturation of the MRI, in many cases the disk maintains a steady vertical structure for many thermal times. However, in every case in which the box size in the horizontal directions are at least one pressure scale height, fluctuations associated with MRI turbulence and dynamo action in the disk eventually trigger a thermal runaway that causes the disk to either expand or contract until the calculation must be terminated. During runaway, the dependence of the heating and cooling rates on total pressure satisfy the simplest criterion for classical thermal instability. We identify several physical reasons why the thermal runaway observed in our simulations differ from the standard α disk model; for example, the advection of radiation contributes a non-negligible fraction to the vertical energy flux at the largest radiation pressure, most of the dissipation does not happen in the disk mid-plane, and the change of dissipation scale height with mid-plane pressure is slower than the change of density scale height. We discuss how and why our results differ from those published previously. Such thermal runaway behavior might have important implications for interpreting temporal variability in observed systems, but fully global simulations are required to study the saturated state before detailed predictions can be made.

  1. On the Thermal Stability of Radiation-dominated Accretion Disks

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei; Stone, James M.; Davis, Shane W.

    2013-11-01

    We study the long-term thermal stability of radiation-dominated disks in which the vertical structure is determined self-consistently by the balance of heating due to the dissipation of MHD turbulence driven by magneto-rotational instability (MRI) and cooling due to radiation emitted at the photosphere. The calculations adopt the local shearing box approximation and utilize the recently developed radiation transfer module in the Athena MHD code based on a variable Eddington tensor rather than an assumed local closure. After saturation of the MRI, in many cases the disk maintains a steady vertical structure for many thermal times. However, in every case in which the box size in the horizontal directions are at least one pressure scale height, fluctuations associated with MRI turbulence and dynamo action in the disk eventually trigger a thermal runaway that causes the disk to either expand or contract until the calculation must be terminated. During runaway, the dependence of the heating and cooling rates on total pressure satisfy the simplest criterion for classical thermal instability. We identify several physical reasons why the thermal runaway observed in our simulations differ from the standard α disk model; for example, the advection of radiation contributes a non-negligible fraction to the vertical energy flux at the largest radiation pressure, most of the dissipation does not happen in the disk mid-plane, and the change of dissipation scale height with mid-plane pressure is slower than the change of density scale height. We discuss how and why our results differ from those published previously. Such thermal runaway behavior might have important implications for interpreting temporal variability in observed systems, but fully global simulations are required to study the saturated state before detailed predictions can be made.

  2. Advection/diffusion of large scale magnetic field in accretion disks

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Bisnovatyi-Kogan, G. S.; Rothstein, D. M.

    2009-02-01

    Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr(z) which depends mainly on the midplane thermal to magnetic pressure ratio β>1 and the Prandtl number of the turbulence P=viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β>1) large-scale field does not diffuse away as suggested by earlier work.

  3. SIMULATIONS OF THE BOUNDARY LAYER BETWEEN A WHITE DWARF AND ITS ACCRETION DISK

    SciTech Connect

    Balsara, Dinshaw S.; Fisker, Jacob Lund; Godon, Patrick; Sion, Edward M. E-mail: jfisker@nd.edu E-mail: edward.sion@villanova.edu

    2009-09-10

    Using a 2.5D time-dependent numerical code we recently developed, we solve the full compressible Navier-Stokes equations to determine the structure of the boundary layer (BL) between the white dwarf (WD) and the accretion disk in nonmagnetic cataclysmic variable systems. In this preliminary work, our numerical approach does not include radiation. In the energy equation, we either take the dissipation function ({phi}) into account or we assume that the energy dissipated by viscous processes is instantly radiated away ({phi} = 0). For a slowly rotating nonmagnetized accreting WD, the accretion disk extends all the way to the stellar surface. There, the matter impacts and spreads toward the poles as new matter continuously piles up behind it. We carry out numerical simulations for different values of the alpha-viscosity parameter ({alpha}), corresponding to different mass accretion rates. In the high viscosity cases ({alpha} = 0.1), the spreading BL sets off a gravity wave in the surface matter. The accretion flow moves supersonically over the cusp making it susceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz shearing instabilities. This BL is optically thick and extends more than 30 deg. to either side of the disk plane after only 3/4 of a Keplerian rotation period (t{sub K} = 19 s). In the low viscosity cases ({alpha} = 0.001), the spreading BL does not set off gravity waves and it is optically thin.

  4. AGN fuelling: Bridging Large and Small Scales - Overlapping Inflows as Catalysts of Accretion

    NASA Astrophysics Data System (ADS)

    Manuel Carmona Loaiza, Juan Manuel

    2015-05-01

    One of the biggest challenges in understanding the fuelling of supermassive black holes in active galactic nuclei (AGN) is not on accounting for the source of fuel, as a galaxy can comfortably supply the required mass budget, but on its actual delivery. While a clear picture has been developed for the large scale (~ kpc) down to the intermediate one (~ 100 pc), and for the smallest scales (~ 0.1 pc) where an accretion disc likely forms, a bridge that has proven difficult to build is that between ~ 100 pc and ~ 0.1 pc. It is feared that gas at these scales might still retain enough angular momentum and settle into a larger scale disc with very low or no inflow to form or replenish the inner accretion disc (on ~ 0.01 pc scales). In this Thesis, I present numerical simulations in which a rotating gaseous shell flows towards a SMBH because of its lack of rotational support. As inflow proceeds, gas from the shell impacts an already present nuclear (~ 10pc) disc. The cancellation of angular momentum and redistribution of gas, due to the misalignment between the angular momentum of the shell and that of the disc, is studied in this scenario. The underlying hypothesis is that even if transport of angular momentum at these scales may be inefficient, the interaction of an inflow with a nuclear disc would still provide a mechanism to bring mass inwards because of the cancellation of angular momentum. I quantify the amount of gas such a cancellation would bring to the central parsec under different circumstances: Co- and counter-rotation between the disc and the shell and the presence or absence of an initial turbulent kick; I also discuss the impact of self gravity in our simulations. The scenario we study is highly idealized and designed to capture the specific outcomes produced by the mechanism proposed. I find that angular momentum cancellation and redistribution via hydrodynamical shocks leads to sub-pc inflows enhanced by more than 2-3 orders of magnitude. In all of our

  5. Angular Momentum Transport in Accretion Disks and Relevance of Thermal Energy Content and Transport

    NASA Astrophysics Data System (ADS)

    Coppi, P. S.; Coppi, B.

    2003-04-01

    Considering thin accretion disks where the magnetic energy is comparable to the thermal energy is relevant to theoretical models of magnetized jets originating from disks. In these regimes the only known modes that can induce outward transport of angular momentum, as needed for the accretion process to occur, are tridimensional, locally corotating with the disk, which, if described by the linearized ideal MHD approximation, have radial profiles with two sets of characteristic singularities(B. Coppi and P.S. Coppi, Phys. Rev. Lett.), 87, 051101-1 (2001). The more important of these two sets corresponds to the radius where the relevant Doppler shifted frequency equals that of the slow magnetosonic mode (ω ^2≃ k_allel^2c_s^2v_A^2/(c_s^2+v_A^2)). Thus the finiteness of the plasma temperature is a key factor in the excitation of these modes. Nonlinear effects are shown to be more important than ``dissipative'' processes to describe the transition across the singularities. The connection between transport of angular momentum and that of thermal energy that is central to the explanation of the ``spontaneous rotation'' phenomenon in laboratory plasmas by the relevant accretion theory(B. Coppi, Nucl. Fusion), 42, 1 (2002) is discussed and the difference of the phsyical regimes of these plasmas relative to those characterizing accretion disks is pointed out.

  6. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni

    NASA Astrophysics Data System (ADS)

    Muñoz-Darias, T.; Casares, J.; Mata Sánchez, D.; Fender, R. P.; Armas Padilla, M.; Linares, M.; Ponti, G.; Charles, P. A.; Mooley, K. P.; Rodriguez, J.

    2016-06-01

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10‑8 solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.

  7. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni

    NASA Astrophysics Data System (ADS)

    Muñoz-Darias, T.; Casares, J.; Mata Sánchez, D.; Fender, R. P.; Armas Padilla, M.; Linares, M.; Ponti, G.; Charles, P. A.; Mooley, K. P.; Rodriguez, J.

    2016-06-01

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10-8 solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.

  8. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni.

    PubMed

    Muñoz-Darias, T; Casares, J; Mata Sánchez, D; Fender, R P; Armas Padilla, M; Linares, M; Ponti, G; Charles, P A; Mooley, K P; Rodriguez, J

    2016-06-01

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10(-8) solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes. PMID:27251277

  9. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni.

    PubMed

    Muñoz-Darias, T; Casares, J; Mata Sánchez, D; Fender, R P; Armas Padilla, M; Linares, M; Ponti, G; Charles, P A; Mooley, K P; Rodriguez, J

    2016-05-09

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10(-8) solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.

  10. The emission lines radiated from accretion disk in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    He, L.; Zhao, Y. H.; Lv, G. L.

    2013-02-01

    During the quiescent phases, the spectra of cataclysmic variables are studied. We model the fluxes by means of the photoionization code CLOUDY, and in the model it is assumed that the geometry of disks is cylindrical. Using this method, we input a series of reasonable parameters and get a spectral library.

  11. Shrinking galaxy disks with fountain-driven accretion from the halo

    SciTech Connect

    Elmegreen, Bruce G.; Struck, Curtis; Hunter, Deidre A. E-mail: curt@iastate.edu

    2014-12-01

    Star formation in most galaxies requires cosmic gas accretion because the gas consumption time is short compared to the Hubble time. This accretion presumably comes from a combination of infalling satellite debris, cold flows, and condensation of hot halo gas at the cool disk interface, perhaps aided by a galactic fountain. In general, the accretion will have a different specific angular momentum than the part of the disk that receives it, even if the gas comes from the nearby halo. The gas disk then expands or shrinks over time. Here we show that condensation of halo gas at a rate proportional to the star formation rate in the fountain model will preserve an initial shape, such as an exponential, with a shrinking scale length, leaving behind a stellar disk with a slightly steeper profile of younger stars near the center. This process is slow for most galaxies, producing imperceptible radial speeds, and it may be dominated by other torques, but it could be important for blue compact dwarfs, which tend to have large, irregular gas reservoirs and steep blue profiles in their inner stellar disks.

  12. Accretion Disks Around Binary Black Holes of Unequal Mass: GRMHD Simulations Near Decoupling

    NASA Technical Reports Server (NTRS)

    Gold, Roman; Paschalidis, Vasileios; Etienne, Zachariah B.; Shapiro, Stuart L.; Pfeiffer, Harald, P.

    2013-01-01

    We report on simulations in general relativity of magnetized disks onto black hole binaries. We vary the binary mass ratio from 1:1 to 1:10 and evolve the systems when they orbit near the binary disk decoupling radius. We compare (surface) density profiles, accretion rates (relative to a single, non-spinning black hole), variability, effective alpha-stress levels and luminosities as functions of the mass ratio. We treat the disks in two limiting regimes: rapid radiative cooling and no radiative cooling. The magnetic field lines clearly reveal jets emerging from both black hole horizons and merging into one common jet at large distances. The magnetic fields give rise to much stronger shock heating than the pure hydrodynamic flows, completely alter the disk structure, and boost accretion rates and luminosities. Accretion streams near the horizons are among the densest structures; in fact, the 1:10 no-cooling evolution results in a refilling of the cavity. The typical effective temperature in the bulk of the disk is approx. 10(exp5) (M / 10(exp 8)M solar mass (exp -1/4(L/L(sub edd) (exp 1/4K) yielding characteristic thermal frequencies approx. 10 (exp 15) (M /10(exp 8)M solar mass) (exp -1/4(L/L (sub edd) (1+z) (exp -1)Hz. These systems are thus promising targets for many extragalactic optical surveys, such as LSST, WFIRST, and PanSTARRS.

  13. Wind from the black-hole accretion disk driving a molecular outflow in an active galaxy

    NASA Astrophysics Data System (ADS)

    Tombesi, F.; Meléndez, M.; Veilleux, S.; Reeves, J. N.; González-Alfonso, E.; Reynolds, C. S.

    2015-03-01

    Powerful winds driven by active galactic nuclei are often thought to affect the evolution of both supermassive black holes and their host galaxies, quenching star formation and explaining the close relationship between black holes and galaxies. Recent observations of large-scale molecular outflows in ultraluminous infrared galaxies support this quasar-feedback idea, because they directly trace the gas from which stars form. Theoretical models suggest that these outflows originate as energy-conserving flows driven by fast accretion-disk winds. Proposed connections between large-scale molecular outflows and accretion-disk activity in ultraluminous galaxies were incomplete because no accretion-disk wind had been detected. Conversely, studies of powerful accretion-disk winds have until now focused only on X-ray observations of local Seyfert galaxies and a few higher-redshift quasars. Here we report observations of a powerful accretion-disk wind with a mildly relativistic velocity (a quarter that of light) in the X-ray spectrum of IRAS F11119+3257, a nearby (redshift 0.189) optically classified type 1 ultraluminous infrared galaxy hosting a powerful molecular outflow. The active galactic nucleus is responsible for about 80 per cent of the emission, with a quasar-like luminosity of 1.5 × 1046 ergs per second. The energetics of these two types of wide-angle outflows is consistent with the energy-conserving mechanism that is the basis of the quasar feedback in active galactic nuclei that lack powerful radio jets (such jets are an alternative way to drive molecular outflows).

  14. Truncation of the Inner Accretion Disk Around a Black Hole at Low Luminosity

    NASA Technical Reports Server (NTRS)

    Tomsick, John A.; Yamaoka, Kazutaka; Corbel, Stephane; Kaaret, Philip; Kalemci, Emrah; Migliari, Simone

    2009-01-01

    Most black hole binaries show large changes in X-ray luminosity caused primarily by variations in mass accretion rate. An important question for understanding black hole accretion and jet production is whether the inner edge of the accretion disk recedes at low accretion rate. Measurements of the location of the inner edge (R(sub in)) can be made using iron emission lines that arise due to fluorescence of iron in the disk, and these indicate that R(sub in) is very close to the black hole at high and moderate luminosities (greater than approximately equal to 1% of the Eddington luminosity, L(sub Edd). Here, we report on X-ray observation of the black hole GX 339-4 in the hard state by Suzaku and the Rossi X-ray Timing Explorer (RXTE) that extend iron line studies to 0.14% L(sub Edd) and show that R(sub in) increases by a factor of greater than 27 over the value found when GX 339-4 was bright. The exact value of R(sub in) depends on the inclination of the inner disk (i), and we derive 90% confidence limits of R(sub in) greater than 35R(sub g) at i = 0 degrees and R(sub in) greater than 175R(sub g) at i = 30 degrees. This provides direct evidence that the inner portion of the disk is not present at low luminosity, allowing for the possibility that the inner disk is replaced by advection- or magnetically-dominated accretion flows.

  15. Truncation of the Inner Accretion Disk Around a Black Hole at Low Luminosity

    NASA Technical Reports Server (NTRS)

    Tomsick, John A.; Yamoka, Kazutaka; Corbel, Stephane; Kaaret, Philip; Kalemci, Emrah; Migliari, Simone

    2011-01-01

    Most black hole binaries show large changes in X-ray luminosity caused primarily by variations in mass accretion rate. An important question for understanding black hole accretion and jet production is whether the inner edge of the accretion disk recedes at low accretion rate. Measurements of the location of the inner edge (R(sub in)) can be made using iron emission lines that arise due to fluorescence of iron in the disk, and these indicate that R(sub in) is very close to the black hole at high and moderate luminosities (greater than or equal to 1% of the Eddington luminosity, L(sub Edd). Here, we report on X-ray observations of the black hole GX 339-4 in the hard state by Suzaku and the Rossi X-ray Timing Explorer that extend iron line studies to 0.14% L(sub Edd) and show that R(sub in) increases by a factor of greater than 27 over the value found when GX 339-4 was bright. The exact value of R(sub in) depends on the inclination of the inner disk (i), and we derive 90% confidence limits of R(sub in) greater than 35 R(sub g) at i = 0 degrees and R(sub in) greater than 175 R(sub g) at i = 30 degrees. This provides direct evidence that the inner portion of the disk is not present at low luminosity, allowing for the possibility that the inner disk is replaced by advection- or magnetically dominated accretion flows.

  16. Surface Layer Accretion in Transitional and Conventional Disks: From Polycyclic Aromatic Hydrocarbons to Planets

    NASA Astrophysics Data System (ADS)

    Perez-Becker, Daniel; Chiang, Eugene

    2011-01-01

    "Transitional" T Tauri disks have optically thin holes with radii gsim10 AU, yet accrete up to the median T Tauri rate. Multiple planets inside the hole can torque the gas to high radial speeds over large distances, reducing the local surface density while maintaining accretion. Thus multi-planet systems, together with reductions in disk opacity due to grain growth, can explain how holes can be simultaneously transparent and accreting. There remains the problem of how outer disk gas diffuses into the hole. Here it has been proposed that the magnetorotational instability (MRI) erodes disk surface layers ionized by stellar X-rays. In contrast to previous work, we find that the extent to which surface layers are MRI-active is limited not by ohmic dissipation but by ambipolar diffusion, the latter measured by Am: the number of times a neutral hydrogen molecule collides with ions in a dynamical time. Simulations by Hawley & Stone showed that Am ~ 100 is necessary for ions to drive MRI turbulence in neutral gas. We calculate that in X-ray-irradiated surface layers, Am typically varies from ~10-3 to 1, depending on the abundance of charge-adsorbing polycyclic aromatic hydrocarbons, whose properties we infer from Spitzer observations. We conclude that ionization of H2 by X-rays and cosmic rays can sustain, at most, only weak MRI turbulence in surface layers 1-10 g cm-2 thick, and that accretion rates in such layers are too small compared to observed accretion rates for the majority of disks.

  17. Wind from the black-hole accretion disk driving a molecular outflow in an active galaxy.

    PubMed

    Tombesi, F; Meléndez, M; Veilleux, S; Reeves, J N; González-Alfonso, E; Reynolds, C S

    2015-03-26

    Powerful winds driven by active galactic nuclei are often thought to affect the evolution of both supermassive black holes and their host galaxies, quenching star formation and explaining the close relationship between black holes and galaxies. Recent observations of large-scale molecular outflows in ultraluminous infrared galaxies support this quasar-feedback idea, because they directly trace the gas from which stars form. Theoretical models suggest that these outflows originate as energy-conserving flows driven by fast accretion-disk winds. Proposed connections between large-scale molecular outflows and accretion-disk activity in ultraluminous galaxies were incomplete because no accretion-disk wind had been detected. Conversely, studies of powerful accretion-disk winds have until now focused only on X-ray observations of local Seyfert galaxies and a few higher-redshift quasars. Here we report observations of a powerful accretion-disk wind with a mildly relativistic velocity (a quarter that of light) in the X-ray spectrum of IRAS F11119+3257, a nearby (redshift 0.189) optically classified type 1 ultraluminous infrared galaxy hosting a powerful molecular outflow. The active galactic nucleus is responsible for about 80 per cent of the emission, with a quasar-like luminosity of 1.5 × 10(46) ergs per second. The energetics of these two types of wide-angle outflows is consistent with the energy-conserving mechanism that is the basis of the quasar feedback in active galactic nuclei that lack powerful radio jets (such jets are an alternative way to drive molecular outflows). PMID:25810204

  18. Wind from the black-hole accretion disk driving a molecular outflow in an active galaxy.

    PubMed

    Tombesi, F; Meléndez, M; Veilleux, S; Reeves, J N; González-Alfonso, E; Reynolds, C S

    2015-03-26

    Powerful winds driven by active galactic nuclei are often thought to affect the evolution of both supermassive black holes and their host galaxies, quenching star formation and explaining the close relationship between black holes and galaxies. Recent observations of large-scale molecular outflows in ultraluminous infrared galaxies support this quasar-feedback idea, because they directly trace the gas from which stars form. Theoretical models suggest that these outflows originate as energy-conserving flows driven by fast accretion-disk winds. Proposed connections between large-scale molecular outflows and accretion-disk activity in ultraluminous galaxies were incomplete because no accretion-disk wind had been detected. Conversely, studies of powerful accretion-disk winds have until now focused only on X-ray observations of local Seyfert galaxies and a few higher-redshift quasars. Here we report observations of a powerful accretion-disk wind with a mildly relativistic velocity (a quarter that of light) in the X-ray spectrum of IRAS F11119+3257, a nearby (redshift 0.189) optically classified type 1 ultraluminous infrared galaxy hosting a powerful molecular outflow. The active galactic nucleus is responsible for about 80 per cent of the emission, with a quasar-like luminosity of 1.5 × 10(46) ergs per second. The energetics of these two types of wide-angle outflows is consistent with the energy-conserving mechanism that is the basis of the quasar feedback in active galactic nuclei that lack powerful radio jets (such jets are an alternative way to drive molecular outflows).

  19. Global MHD Simulations of Accretion Disks in Cataclysmic Variables. I. The Importance of Spiral Shocks

    NASA Astrophysics Data System (ADS)

    Ju, Wenhua; Stone, James M.; Zhu, Zhaohuan

    2016-06-01

    We present results from the first global 3D MHD simulations of accretion disks in cataclysmic variable (CV) systems in order to investigate the relative importance of angular momentum transport via turbulence driven by the magnetorotational instability (MRI) compared with that driven by spiral shock waves. Remarkably, we find that even with vigorous MRI turbulence, spiral shocks are an important component of the overall angular momentum budget, at least when temperatures in the disk are high (so that Mach numbers are low). In order to understand the excitation, propagation, and damping of spiral density waves in our simulations more carefully, we perform a series of 2D global hydrodynamical simulations with various equation of states, both with and without mass inflow via the Lagrangian point (L1). Compared with previous similar studies, we find the following new results. (1) The linear wave dispersion relation fits the pitch angles of spiral density waves very well. (2) We demonstrate explicitly that mass accretion is driven by the deposition of negative angular momentum carried by the waves when they dissipate in shocks. (3) Using Reynolds stress scaled by gas pressure to represent the effective angular momentum transport rate {α }{eff} is not accurate when mass accretion is driven by non-axisymmetric shocks. (4) Using the mass accretion rate measured in our simulations to directly measure α defined in standard thin-disk theory, we find 0.02≲ {α }{eff}≲ 0.05 for CV disks, consistent with observed values in quiescent states of dwarf novae. In this regime, the disk may be too cool and neutral for the MRI to operate and spiral shocks are a possible accretion mechanism. However, we caution that our simulations use unrealistically low Mach numbers in this regime and, therefore, future models with more realistic thermodynamics and non-ideal MHD are warranted.

  20. Accretion Disk Signatures in Type I X-Ray Bursts: Prospects for Future Missions

    NASA Astrophysics Data System (ADS)

    Keek, L.; Wolf, Z.; Ballantyne, D. R.

    2016-07-01

    Type I X-ray bursts and superbursts from accreting neutron stars illuminate the accretion disk and produce a reflection signal that evolves as the burst fades. Examining the evolution of reflection features in the spectra will provide insight into the burst-disk interaction, a potentially powerful probe of accretion disk physics. At present, reflection has been observed during only two bursts of exceptional duration. We investigate the detectability of reflection signatures with four of the latest well-studied X-ray observatory concepts: Hitomi, Neutron Star Interior Composition Explorer (NICER), Athena, and Large Observatory For X-ray Timing (LOFT). Burst spectra are modeled for different values for the flux, temperature, and the disk ionization parameter, which are representative for most known bursts and sources. The effective area and throughput of a Hitomi-like telescope are insufficient for characterizing burst reflection features. NICER and Athena will detect reflection signatures in Type I bursts with peak fluxes ≳10-7.5 erg cm-2 s-1 and also effectively constrain the reflection parameters for bright bursts with fluxes of ˜10-7 erg cm-2 s-1 in exposures of several seconds. Thus, these observatories will provide crucial new insight into the interaction of accretion flows and X-ray bursts. For sources with low line-of-sight absorption, the wide bandpass of these instruments allows for the detection of soft X-ray reflection features, which are sensitive to the disk metallicity and density. The large collecting area that is part of the LOFT design would revolutionize the field by tracing the evolution of the accretion geometry in detail throughout short bursts.

  1. A GENERAL RELATIVISTIC MODEL OF ACCRETION DISKS WITH CORONAE SURROUNDING KERR BLACK HOLES

    SciTech Connect

    You Bei; Cao Xinwu; Yuan Yefei E-mail: cxw@shao.ac.cn

    2012-12-20

    We calculate the structure of a standard accretion disk with a corona surrounding a massive Kerr black hole in the general relativistic frame, in which the corona is assumed to be heated by the reconnection of the strongly buoyant magnetic fields generated in the cold accretion disk. The emergent spectra of accretion disk-corona systems are calculated by using the relativistic ray-tracing method. We propose a new method to calculate the emergent Comptonized spectra from the coronae. The spectra of disk-corona systems with a modified {alpha}-magnetic stress show that both the hard X-ray spectral index and the hard X-ray bolometric correction factor L{sub bol}/L{sub X,2-10keV} increase with the dimensionless mass accretion rate, which is qualitatively consistent with the observations of active galactic nuclei. The fraction of the power dissipated in the corona decreases with increasing black hole spin parameter a, which leads to lower electron temperatures of the coronae for rapidly spinning black holes. The X-ray emission from the coronae surrounding rapidly spinning black holes becomes weak and soft. The ratio of the X-ray luminosity to the optical/UV luminosity increases with the viewing angle, while the spectral shape in the X-ray band is insensitive to the viewing angle. We find that the spectral index in the infrared waveband depends on the mass accretion rate and the black hole spin a, which deviates from the f{sub {nu}}{proportional_to}{nu}{sup 1/3} relation expected by the standard thin disk model.

  2. Advection/Diffusion of Large-Scale B Field in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Rothstein, D. M.; Bisnovatyi-Kogan, G. S.

    2009-08-01

    Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magnetorotational instability, MRI), which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field, which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity, which acts to prevent the buildup of a significant large-scale field. Recent work has pointed out that the disk's surface layers are nonturbulent, and thus highly conducting (or nondiffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components) and the profiles of the large-scale magnetic field, taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr (z), which depends mainly on the midplane thermal to magnetic pressure ratio β>1 and the Prandtl number of the turbulence P= viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β>1) large-scale field does not diffuse away as suggested by earlier work. For a wide range of parameters β>1 and P≥ 1, we find stationary channel-type flows where the flow is radially outward near the midplane of the disk and radially inward in the top and bottom parts of the disk. Channel flows with inward flow near the midplane and outflow in the top and bottom

  3. Mapping the average AGN accretion rate in the SFR-M* plane for Herschel-selected galaxies at 0 < z ≤ 2.5

    NASA Astrophysics Data System (ADS)

    Delvecchio, I.; Lutz, D.; Berta, S.; Rosario, D. J.; Zamorani, G.; Pozzi, F.; Gruppioni, C.; Vignali, C.; Brusa, M.; Cimatti, A.; Clements, D. L.; Cooray, A.; Farrah, D.; Lanzuisi, G.; Oliver, S.; Rodighiero, G.; Santini, P.; Symeonidis, M.

    2015-05-01

    We study the relation of AGN accretion, star formation rate (SFR) and stellar mass (M*) using a sample of ≈8600 star-forming galaxies up to z = 2.5 selected with Herschel imaging in the GOODS and COSMOS fields. For each of them we derive SFR and M*, both corrected, when necessary, for emission from an active galactic nucleus (AGN), through the decomposition of their spectral energy distributions (SEDs). About 10 per cent of the sample are detected individually in Chandra observations of the fields. For the rest of the sample, we stack the X-ray maps to get average X-ray properties. After subtracting the X-ray luminosity expected from star formation and correcting for nuclear obscuration, we derive the average AGN accretion rate for both detected sources and stacks, as a function of M*, SFR and redshift. The average accretion rate correlates with SFR and with M*. The dependence on SFR becomes progressively more significant at z > 0.8. This may suggest that SFR is the original driver of these correlations. We find that average AGN accretion and star formation increase in a similar fashion with offset from the star-forming `main-sequence'. Our interpretation is that accretion on to the central black hole and star formation broadly trace each other, irrespective of whether the galaxy is evolving steadily on the main-sequence or bursting.

  4. A Global 3D Radiation MHD Simulation of Super-Eddington Accretion Disks

    NASA Astrophysics Data System (ADS)

    Jiang, Yanfei

    2014-10-01

    We study how black holes can accrete above the Eddington limit using a global three dimensional radiation magneto-hydrodynamic simulation without ad-hoc assumptions. The simulation reaches an accretion rate ~ 220L_Edd/c^2 and forms a radiation driven outflow along the rotation axis. The radiative luminosity of this flow is ~ 10L_Edd. This yields a radiative efficiency ~ 4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in slim disk model, which neglect vertical advection. The results have important implications for the growth of supermassive black holes in the early universe, tidal disruption events and ultra-luminous X-ray sources.

  5. Is Transport in Accretion Disks Primarily Local or Non-local?

    NASA Astrophysics Data System (ADS)

    Blackman, Eric G.

    2014-10-01

    Accretion disks likely involve some combination of local and non-local angular momentum transport. Coronae and jets provide evidence for large scale transport and disk thermal emission may provide evidence for local transport. Identifying the principles that determine the relative local vs. nonlocal fraction poses a set of challenges and highlights a significant gap between numerical simulation results and improved, practical mean field accretion theory. The dominant mechanisms of transport may in fact be non-local and non-viscous. Even the magneto-rotational instability (MRI) for example, often invoked as a source of local turbulence, may produce predominantly non-local transport. I will overview progress and open issues on these themes, drawing in concepts from disk theory, dynamo theory, and corona formation.

  6. How to Determine The Precession of the Inner Accretion Disk in Cygnus X-1

    SciTech Connect

    Torres, D F; Romero, G E; Barcons, X; Lu, Y

    2005-01-05

    We show that changes in the orientation of the inner accretion disk of Cygnus X-1 affect the shape of the broad Fe K{alpha} emission line emitted from this object, in such a way that eV-level spectral resolution observations (such as those that will be carried out by the ASTRO-E2 satellite) can be used to analyze the dynamics of the disk. We here present a new diagnosis tool, supported by numerical simulations, by which short observations of Cygnus X-1, separated in time, can determine whether its accretion disk actually processes, and if so, determine its period and precession angle. Knowing the precession parameters of Cygnus X-1 would result in a clarification of the origin of such precession, distinguishing between tidal and spin-spin coupling. This approach could also be used for similar studies in other microquasar systems.

  7. Gas content of transitional disks: a VLT/X-Shooter study of accretion and winds

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Testi, L.; Natta, A.; Rosotti, G.; Benisty, M.; Ercolano, B.; Ricci, L.

    2014-08-01

    Context. Transitional disks are thought to be a late evolutionary stage of protoplanetary disks whose inner regions have been depleted of dust. The mechanism responsible for this depletion is still under debate. To constrain the various models it is mandatory to have a good understanding of the properties of the gas content in the inner part of the disk. Aims: Using X-Shooter broad band - UV to near-infrared - medium-resolution spectroscopy, we derive the stellar, accretion, and wind properties of a sample of 22 transitional disks. The analysis of these properties allows us to place strong constraints on the gas content in a region very close to the star (≲0.2 AU) that is not accessible with any other observational technique. Methods: We fitted the spectra with a self-consistent procedure to simultaneously derive spectral type, extinction, and accretion properties of the targets. From the continuum excess at near-infrared wavelength we distinguished whether our targets have dust free inner holes. By analyzing forbidden emission lines, we derived the wind properties of the targets. We then compared our findings with results for classical T Tauri stars. Results: The accretion rates and wind properties of 80% of the transitional disks in our sample, which is strongly biased toward stongly accreting objects, are comparable to those of classical T Tauri stars. Thus, there are (at least) some transitional disks with accretion properties compatible with those of classical T Tauri stars, irrespective of the size of the dust inner hole. Only in two cases are the mass accretion rates much lower, while the wind properties remain similar. We detected no strong trend of the mass accretion rates with the size of the dust-depleted cavity or with the presence of a dusty optically thick disk very close to the star. These results suggest that, close to the central star, there is a gas-rich inner disk with a density similar to that of classical T Tauri star disks. Conclusions: The

  8. ON THE ROLE OF DISKS IN THE FORMATION OF STELLAR SYSTEMS: A NUMERICAL PARAMETER STUDY OF RAPID ACCRETION

    SciTech Connect

    Kratter, Kaitlin M.; Matzner, Christopher D.; Krumholz, Mark R.; Klein, Richard I.

    2010-01-10

    We study rapidly accreting, gravitationally unstable disks with a series of idealized global, numerical experiments using the code ORION. Our numerical parameter study focuses on protostellar disks, showing that one can predict disk behavior and the multiplicity of the accreting star system as a function of two dimensionless parameters which compare the infall rate to the disk sound speed and orbital period. Although gravitational instabilities become strong, we find that fragmentation into binary or multiple systems occurs only when material falls in several times more rapidly than the canonical isothermal limit. The disk-to-star accretion rate is proportional to the infall rate and governed by gravitational torques generated by low-m spiral modes. We also confirm the existence of a maximum stable disk mass: disks that exceed approx50% of the total system mass are subject to fragmentation and the subsequent formation of binary companions.

  9. Generalized Langevin equation with colored noise description of the stochastic oscillations of accretion disks

    NASA Astrophysics Data System (ADS)

    Harko, Tiberiu; Leung, Chun Sing; Mocanu, Gabriela

    2014-05-01

    We consider a description of the stochastic oscillations of the general relativistic accretion disks around compact astrophysical objects interacting with their external medium based on a generalized Langevin equation with colored noise and on the fluctuation-dissipation theorems. The former accounts for the general memory and retarded effects of the frictional force. The presence of the memory effects influences the response of the disk to external random interactions, and it modifies the dynamical behavior of the disk, as well as the energy dissipation processes. The generalized Langevin equation of the motion of the disk in the vertical direction is studied numerically, and the vertical displacements, velocities, and luminosities of the stochastically perturbed disks are explicitly obtained for both the Schwarzschild and the Kerr cases. The power spectral distribution of the disk luminosity is also obtained. As a possible astrophysical application of the formalism we investigate the possibility that the intra-day variability of the active galactic nuclei may be due to the stochastic disk instabilities. The perturbations due to colored/nontrivially correlated noise induce a complicated disk dynamics, which could explain some astrophysical observational features related to disk variability.

  10. ACCRETION-INHIBITED STAR FORMATION IN THE WARM MOLECULAR DISK OF THE GREEN-VALLEY ELLIPTICAL GALAXY NGC 3226?

    SciTech Connect

    Appleton, P. N.; Bitsakis, T.; Alatalo, K.; Mundell, C.; Lacy, M.; Armus, L.; Charmandaris, V.; Duc, P.-A.; Lisenfeld, U.; Ogle, P.

    2014-12-20

    We present archival Spitzer photometry and spectroscopy and Herschel photometry of the peculiar ''Green Valley'' elliptical galaxy NGC 3226. The galaxy, which contains a low-luminosity active galactic nucleus (AGN), forms a pair with NGC 3227 and is shown to lie in a complex web of stellar and H I filaments. Imaging at 8 and 16 μm reveals a curved plume structure 3 kpc in extent, embedded within the core of the galaxy and coincident with the termination of a 30 kpc long H I tail. In situ star formation associated with the infrared (IR) plume is identified from narrowband Hubble Space Telescope (HST) imaging. The end of the IR plume coincides with a warm molecular hydrogen disk and dusty ring containing 0.7-1.1 × 10{sup 7} M {sub ☉} detected within the central kiloparsec. Sensitive upper limits to the detection of cold molecular gas may indicate that a large fraction of the H{sub 2} is in a warm state. Photometry derived from the ultraviolet (UV) to the far-IR shows evidence for a low star-formation rate of ∼0.04 M {sub ☉} yr{sup –1} averaged over the last 100 Myr. A mid-IR component to the spectral energy distribution (SED) contributes ∼20% of the IR luminosity of the galaxy, and is consistent with emission associated with the AGN. The current measured star formation rate is insufficient to explain NGC 3226's global UV-optical ''green'' colors via the resurgence of star formation in a ''red and dead'' galaxy. This form of ''cold accretion'' from a tidal stream would appear to be an inefficient way to rejuvenate early-type galaxies and may actually inhibit star formation.

  11. Is the Blazar Sequence related to accretion disk winds?

    NASA Astrophysics Data System (ADS)

    Boula, Stella; Mastichiadis, Apostolos; Kazanas, Demosthenes

    2016-08-01

    Adopting the hypothesis that the nonthermal emission of blazars is primarily due to the acceleration of electrons, we construct a simple leptonic model in order to explain the Blazar Sequence. The acceleration process is assumed to be of the first order Fermi type and the injected electrons and photons in the emitting region of the blazar are described by spatially averaged kinetic equations. According to the leptonic scenario, the spectral energy distributions of blazars have two basic components: a low frequency component, peaking in the optical through X-rays, from synchrotron emission; and a high frequency one, peaking in the γ rays, probably originating from Compton scattering of some seed photon source, either internal (synchrotron self-Compton) and/or external to the jet (external Compton). We find an adequate description of the Blazar Sequence by assuming a wind density profile of the form n(r) 1/r. Higher luminosity objects have higher accretion rates, higher optical thicknesses of the wind to Compton scattering and thus higher external photon fields than the lower luminosity ones. Therefore, we present indicative Blazar Sequence models which reproduce the basic observational trends just by varying one parameter, namely the mass accretion rate dot{m}.

  12. BIPOLAR JETS LAUNCHED FROM MAGNETICALLY DIFFUSIVE ACCRETION DISKS. I. EJECTION EFFICIENCY VERSUS FIELD STRENGTH AND DIFFUSIVITY

    SciTech Connect

    Sheikhnezami, Somayeh; Fendt, Christian; Porth, Oliver; Vaidya, Bhargav; Ghanbari, Jamshid E-mail: fendt@mpia.de

    2012-09-20

    We investigate the launching of jets and outflows from magnetically diffusive accretion disks. Using the PLUTO code, we solve the time-dependent resistive magnetohydrodynamic equations taking into account the disk and jet evolution simultaneously. The main question we address is which kind of disks launch jets and which kind of disks do not? In particular, we study how the magnitude and distribution of the (turbulent) magnetic diffusivity affect mass loading and jet acceleration. We apply a turbulent magnetic diffusivity based on {alpha}-prescription, but also investigate examples where the scale height of diffusivity is larger than that of the disk gas pressure. We further investigate how the ejection efficiency is governed by the magnetic field strength. Our simulations last for up to 5000 dynamical timescales corresponding to 900 orbital periods of the inner disk. As a general result, we observe a continuous and robust outflow launched from the inner part of the disk, expanding into a collimated jet of superfast-magnetosonic speed. For long timescales, the disk's internal dynamics change, as due to outflow ejection and disk accretion the disk mass decreases. For magnetocentrifugally driven jets, we find that for (1) less diffusive disks, (2) a stronger magnetic field, (3) a low poloidal diffusivity, or (4) a lower numerical diffusivity (resolution), the mass loading of the outflow is increased-resulting in more powerful jets with high-mass flux. For weak magnetization, the (weak) outflow is driven by the magnetic pressure gradient. We consider in detail the advection and diffusion of magnetic flux within the disk and we find that the disk and outflow magnetization may substantially change in time. This may have severe impact on the launching and formation process-an initially highly magnetized disk may evolve into a disk of weak magnetization which cannot drive strong outflows. We further investigate the jet asymptotic velocity and the jet rotational velocity in

  13. Escape conditions of radiative-driven strati from luminous accretion disks

    NASA Astrophysics Data System (ADS)

    Nakai, Takuya; Fukue, Jun

    2015-10-01

    We examine the dynamical motion and escape conditions of continuum-driven strati (flat cloud) with finite optical depth from luminous accretion disks around a black hole. We adopt the near-disk approximation, and treat the problem in the framework of special relativity, where the radiation drag force as well as the radiation pressure are included. We find that the optically thin strati are easy to accelerate, compared with the particles, and the escape condition of the stratus is reduced. That is, when the disk luminosity is around the Eddington luminosity, the optically thin strati can escape from the inner disk (≲ 20 rg; rg being the Schwarzschild radius). When the disk luminosity is about half the Eddington luminosity, it can escape at around 5 rg. This is due to the translucent effect. In addition, the trajectories of the strati are not vertical, but a funnel-like shape due to the centrifugal force. Stratus outflow could easily blow out from usual accretion disks with sub-Eddington luminosities, and this may explain outflows observed in broad absorption line quasars and ultra-fast outflow objects.

  14. Detailed Mid- and Far- Ultraviolet Model Spectra for Accretion Disks in Cataclysmic Binaries

    NASA Technical Reports Server (NTRS)

    Wade, Richard A.; Hubeny, Ivan

    1998-01-01

    We present a large grid of computed far- and mid-ultraviolet spectra (850-2000 A) of the integrated light from steady-state accretion disks in luminous cataclysmic variables. The spectra are tabulated at 0.25 A intervals with an adopted FWHM resolution of 1.0 A, so they are suitable for use with observed spectra from a variety of modern space-borne observatories. Twenty-six different combinations of white dwarf mass M(sub wd) and mass accretion rate dot-m are considered, and spectra are presented for six different disk inclinations i. The disk models are computed self-consistently in the plane-parallel approximation, assuming LTE and vertical hydrostatic equilibrium, by solving simultaneously the radiative transfer, hydrostatic equilibrium, and energy balance equations. Irradiation from external sources is neglected. Local spectra of disk annuli are computed taking into account line transitions from elements 1-28 (H through Ni). Limb darkening as well as Doppler broadening and blending of lines are taken into account in computing the integrated disk spectra. The radiative properties of the models are discussed, including the dependence of ultraviolet fluxes and colors on M(sub wd), dot-m, and i. The appearance of the disk spectra is illustrated, with regard to changes in the same three parameters. Finally, possible future improvements to the present models and spectra are discussed.

  15. NUCLEOSYNTHESIS IN THE OUTFLOWS ASSOCIATED WITH ACCRETION DISKS OF TYPE II COLLAPSARS

    SciTech Connect

    Banerjee, Indrani; Mukhopadhyay, Banibrata E-mail: bm@physics.iisc.ernet.in

    2013-11-20

    We investigate nucleosynthesis inside the outflows from gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, massive stars undergo core collapse to form a proto-neutron star initially, and a mild supernova (SN) explosion is driven. The SN ejecta lack momentum, and subsequently this newly formed neutron star gets transformed to a stellar mass black hole via massive fallback. The hydrodynamics and the nucleosynthesis in these accretion disks have been studied extensively in the past. Several heavy elements are synthesized in the disk, and much of these heavy elements are ejected from the disk via winds and outflows. We study nucleosynthesis in the outflows launched from these disks by using an adiabatic, spherically expanding outflow model, to understand which of these elements thus synthesized in the disk survive in the outflow. While studying this, we find that many new elements like isotopes of titanium, copper, zinc, etc., are present in the outflows. {sup 56}Ni is abundantly synthesized in most of the cases in the outflow, which implies that the outflows from these disks in a majority of cases will lead to an observable SN explosion. It is mainly present when outflow is considered from the He-rich, {sup 56}Ni/{sup 54}Fe-rich zones of the disks. However, outflow from the Si-rich zone of the disk remains rich in silicon. Although emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs by Chandra, BeppoSAX, XMM-Newton, etc., Swift seems to have not yet detected these lines.

  16. RADIAL TRANSPORT OF LARGE-SCALE MAGNETIC FIELDS IN ACCRETION DISKS. II. RELAXATION TO STEADY STATES

    SciTech Connect

    Takeuchi, Taku; Okuzumi, Satoshi

    2014-12-20

    We study the time evolution of a large-scale magnetic flux threading an accretion disk. The induction equation of the mean poloidal field is solved under the standard viscous disk model. Magnetic flux evolution is controlled by two timescales: one is the timescale of the inward advection of the magnetic flux, τ{sub adv}. This is induced by the dragging of the flux by the accreting gas. The other is the outward diffusion timescale of the magnetic flux τ{sub dif}. We consider diffusion due to the Ohmic resistivity. These timescales can be significantly different from the disk viscous timescale τ{sub disk}. The behaviors of the magnetic flux evolution are quite different depending on the magnitude relationship of the timescales τ{sub adv}, τ{sub dif}, and τ{sub disk}. The most interesting phenomena occur when τ{sub adv} << τ{sub dif}, τ{sub disk}. In such a case, the magnetic flux distribution approaches a quasi-steady profile much faster than the viscous evolution of the gas disk, and the magnetic flux has also been tightly bundled to the inner part of the disk. In the inner part, although the poloidal magnetic field becomes much stronger than the interstellar magnetic field, the field strength is limited to the maximum value that is analytically given by our previous work. We also find a condition for the initial large magnetic flux, which is a fossil of the magnetic field dragging during the early phase of star formation that survives for a duration in which significant gas disk evolution proceeds.

  17. High-Resolution N-Body Simulation of Lunar Accretion from an Impact-Generated Disk

    NASA Astrophysics Data System (ADS)

    Kokubo, E.; Ida, S.; Makino, J.

    1998-01-01

    We present the high-resolution N-body calculation of the lunar accretion from a circumterrestrial disk of debris generated by a giant impact on Earth. Our calculation is inspired by pioneering work, which was the first published N-body calculation of lunar accretion from an impact-generated disk. They modeled the disk by 1000 1500 particles, which were not enough to see the evolution of the spatial structure of the disk. By using the special-purpose computer for N-body problem HARP, we use 10,000 30,000 particles for the disk. The particle disk is modeled by the power-law surface density distribution and the power-law mass distribution. We use the 4th- order Hermite integrator for time integration. Mutual gravitational interactions among all particles are calculated by HARP. The normal and tangential components of the coefficient of restitution are 0.01-0.1 and 1, respectively. When two particles collide, we merge them if their rebound velocity is less than their surface escape velocity and the sum of their radii is smaller than 70% of their Hill radii, which corresponds to the fact that the two particles are outside their Roche limit. We also perform simulations without artificial accretion in which large bound particle aggregates are formed outside the Roche limit. The aggregates are often deformed or sometimes disrupted by Earth's tidal force. The results of our simulations suggest that the approximate treatment of accretion is generally valid. We follow the evolution of the disk for 1000 TK about 10 months, where TK is the Kepler period at the distance of the classical Roche limit. The typical outcome of the simulation is the formation of one large moon on a nearly circular orbit just outside the Roche limit. The result hardly depends on the initial condition of the disk as long as the disk is compact; in other words, most of the disk's mass exists inside the Roche limit. The evolution of f the dis is summarized as follows: (1) The disk shrinks through the

  18. Time Series Analysis of the UV Flickering in AGN

    NASA Technical Reports Server (NTRS)

    Robinson, Edward L.

    2003-01-01

    Goals of the Research: Many active galactic nuclei (AGN) exhibit large-amplitude luminosity fluctuations on short timescales. The fluctuations lead to a profound conclusion: The size of the emitting region is remarkably small. This observational fact is one of the pillars supporting the AGN paradigm: Prodigious amounts of gravitational potential energy are liberated in an accretion disk around a supermassive black hole. The goals of the research were to extract from the IUE Archive the very best observational characterizations of AGN flickering, and to use these to test and develop models for AGN variability.

  19. The Bright Side of Black Holes: Radiation from Black Hole Accretion Disks

    NASA Astrophysics Data System (ADS)

    Zhu, Yucong

    An understanding of radiation is paramount for connecting observations of accretion disks with the theory of black holes. In this thesis, we explore via radiative transfer postprocessing calculations the observational signatures of black holes. We investigate disk spectra by analyzing general relativistic magnetohydrodynamic (GRMHD) simulations of accretion disks. For the most part there are no surprises -- the resulting GRMHD spectrum is very close to the analytic Novikov & Thorne (1973) prediction from decades past, except for a small modification in the case of spinning black holes, which exhibit a high-energy power-law tail that is sourced by hot Comptonized gas from within the plunging region of the accretion flow. These conclusions are borne out by both 1D and 3D radiative transfer calculations of the disk. Significant effort was spent in developing from scratch the 3D radiative code that we used for the analysis. The code is named HERO (Hybrid Evaluator for Radiative Objects) and it is a new general purpose grid-based 3D general relativistic radiative solver.

  20. Lunar volatile depletion due to incomplete accretion within an impact-generated disk

    NASA Astrophysics Data System (ADS)

    Canup, Robin M.; Visscher, Channon; Salmon, Julien; Fegley, Bruce

    2015-11-01

    The Moon likely formed from a disk produced by a giant impact with the Earth. The Moon and the bulk silicate Earth (BSE) share many compositional similarities (e.g., Ringwood 1979; Dauphas et al. 2014). However compared with the BSE, the Moon is more depleted in volatile elements, including moderately volatile K and Na, as well as more highly volatile elements, e.g., Zn (e.g., O’Neill 1991; Taylor & Wieczorek 2014). The origin of this depletion is poorly understood. Prior results suggest escape (e.g., Paniello et al. 2012), but at least hydrodynamic escape appears minimal for expected disk conditions (Nakajima & Stevenson 2014).In the limit of no escape and a closed system, a depletion could instead result if disk volatiles were preferentially accreted by the Earth rather than by the Moon. Taylor et al. (2006) advocated that the lunar depletion pattern is most consistent with incomplete condensation from an initially high temperature vapor, with the accretion of condensates by the Moon “cut-off” at a temperature allowing incorporation of a small component of alkalis (e.g., K and Na) but only a tiny fraction of more volatile elements (e.g., Zn). Neither the mechanism that would produce the cut-off, nor what the relevant cut-off temperature would be in an oxygen-rich protolunar disk (e.g., Visscher & Fegley 2013), were known.We identify a mechanism wherein a depletion results because disk volatiles are preferentially accreted by the Earth rather than by the Moon. The Moon may acquire the final tens to 60% of its mass from melt originating from the inner portions of the disk (Salmon & Canup 2012). Initially the inner disk melt is hot and volatile-poor, but as the disk cools, volatiles condense. We combine dynamical, thermal and chemical models to show that delivery of inner disk material to the Moon effectively ends as gravitational interactions cause the Moon’s orbit to expand away from the disk, with this cut-off occurring prior to condensation of key

  1. TIME-VARIABLE ACCRETION IN THE TW Hya STAR/DISK SYSTEM

    SciTech Connect

    Eisner, J. A.; McCarthy, D.; Kulesa, C.; Swift, B. J.; Teske, J.; Doppmann, G. W.; Najita, J. R.

    2010-10-10

    We present two epochs of observations of TW Hya from the high-dispersion near-IR spectrograph ARIES at the Multiple Mirror Telescope. We detect strong emission from the Br{gamma} transition of hydrogen, indicating an accretion rate substantially larger than previously estimated, using hydrogen line emission. The Br{gamma} line strength varies across our two observed epochs. We also measure circumstellar-to-stellar flux ratios (i.e., veilings) that appear close to zero in both epochs. These findings suggest that TW Hya experiences episodes of enhanced accretion while the inner disk remains largely devoid of dust. We discuss several physical mechanisms that may explain these observations.

  2. Mass loss from pre-main-sequence accretion disks. I - The accelerating wind of FU Orionis

    NASA Technical Reports Server (NTRS)

    Calvet, Nuria; Hartmann, Lee; Kenyon, Scott J.

    1993-01-01

    We present evidence that the wind of the pre-main-sequence object FU Orionis arises from the surface of the luminous accretion disk. A disk wind model calculated assuming radiative equilibrium explains the differential behavior of the observed asymmetric absorption-line profiles. The model predicts that strong lines should be asymmetric and blueshifted, while weak lines should be symmetric and double-peaked due to disk rotation, in agreement with observations. We propose that many blueshifted 'shell' absorption features are not produced in a true shell of material, but rather form in a differentially expanding wind that is rapidly rotating. The inference of rapid rotation supports the proposal that pre-main-sequence disk winds are rotationally driven.

  3. Is the Oort A-value a universal growth rate limit for accretion disk shear instabilities?

    NASA Technical Reports Server (NTRS)

    Balbus, Steven A.; Hawley, John F.

    1992-01-01

    A weak-field local MHD instability that is of importance to accretion disks is examined. The maximum growth rate of the instability is found to be not only independent of the magnetic field strength but independent of field geometry as well. In particular, all Keplerian disks are unstable in the presence of any weak poloidal field, with the ratio of the maximum growth rate to disk angular velocity given by 3/4. The maximum growth rate of any weak field configuration that is not purely toroidal is given by the local Oort A-value of the disk. The behavior is studied by using a form of the dynamical Hill equations. It is conjectured that the Oort A-value is an upper bound to the growth rate of any instability feeding upon the free energy of differential rotation.

  4. Accretion Disk Structure in Various Spectral States of GRS 1915+105

    NASA Astrophysics Data System (ADS)

    Remillard, Ronald

    2000-09-01

    GRS 1915+105 displays 9 types of light curves that fall in 3 categories. In the steady-hard states, the Fe line is strongest, and there is a steady type of jet. In the soft states, the accretion disk dominates the X-ray spectrum, and we often detect the 67 Hz QPO thought to arise from GR effects in the inner disk. The remaining states show a variety of instability oscillations, some producing violent mass ejections. Progress on all fronts requires high resolution spectra to help interpret the disk structure. We have particular interest in the profiles of broad Fe emission, intending to gain physical insights using theoretical models of Nayakshin et al. With monitoring timescales selected to randomize the 9 states, we request 3 obs likely to sample different conditions in the disk.

  5. The Origin of Warped, Precessing Accretion Disks in X-ray Binaries

    NASA Technical Reports Server (NTRS)

    Maloney, Philip R.; Begelman, Mitchell C.

    1997-01-01

    The radiation-driven warping instability discovered by Pringle holds considerable promise as the mechanism responsible for producing warped, precessing accretion disks in X-ray binaries. This instability is an inherently global mode of the disk, thereby avoiding the difficulties with earlier models for the precession. Here we follow up on earlier work to study the linear behavior of the instability in the specific context of a binary system. We treat the influence of the companion as an orbit-averaged quadrupole torque on the disk. The presence of this external torque allows the existence of solutions in which the direction of precession of the warp is retrograde with respect to disk rotation, in addition to the prograde solutions that exist in the absence of external torques.

  6. Accretion disks in the IRAS 23151+5912 region

    SciTech Connect

    Rodríguez-Esnard, T.; Trinidad, M. A.; Migenes, V. E-mail: trinidad@astro.ugto.mx

    2014-06-20

    We present observations of radio continuum emission at 1.3 and 3.6 cm and H{sub 2}O masers toward the high-mass star-forming region IRAS 23151+5912 carried out with the Very Large Array-Expanded Very Large Array (in transition phase) in configuration A. We detected one continuum source at 1.3 cm in the region, but the counterpart of this radio continuum source at 3.6 cm was not detected at a level of 3σ. We also detected 13 water maser spots toward IRAS 23151+5912, which are distributed in three groups aligned along the northeast-southwest direction. Our results suggest that the 1.3 cm emission is consistent with a hypercompact H II region, probably with an embedded zero-age main-sequence star of type B2. In particular, we find that this young star is spatially associated with a maser group, which is tracing a disk-like structure of about 460 AU. We also find that the masers of the second group are probably describing a circumstellar disk of about 86 AU, whose central protostar, still undetected, should have a mass of ∼11 M {sub ☉}. We also suggest that the third water maser group is possibly associated with the SiO outflow and the undetected driving source of the system. Finally, we noted that the 1.3 cm continuum source and the three maser groups are aligned in the northeast-southwest direction, similar to the elongation of the large ionized region, which probably is the result of shock-wave induced star formation on the densest region of the medium.

  7. The Accretion Disk and White Dwarf in the Dwarf Nova HS1804+6753

    NASA Astrophysics Data System (ADS)

    Marsh, Thomas

    1996-07-01

    We will take high-speed UV spectroscopy of the dwarf novaHS1804+6753 to find and measure the role of supersonic turbulencein its outer accretion disk. With HST observations of theeclipsing dwarf nova OY Car we have discovered a sensitiveprobe of physical conditions in its accretion disk. This isbased upon model atmosphere fits to strong veiling of thewhite dwarf by a mass of FeII lines at ultravioletwavelengths. Suprisingly we found supersonic microturblence atMach ~ 6 in the absorbing gas. While this would normally beexpected to dissipate in shocks, recent theoretical worksuggests that the disk can become strongly magnetic in whichcase it is the Alfven rather than sound speed which isrelevant. The result needs to be confirmed on other targets.Other systems are also needed to study vertical stratificationin the disk as a single object only gives us one line ofsight. Eclipsing dwarf novae are of particular interest as theeclipse allows the component spectra to be separated andbecause the line-of-sight is likely to pass through the disk.Therefore we propose to observe a newly discovered andrelatively bright eclipsing dwarf nova named HS1804+6753. At 5hours HS1804+6753 has a much longer period than any other suitabletarget and there are signs that it has a particularly highinclination and therefore it will provide an extreme test ofour model.

  8. A model of an X-ray-illuminated accretion disk and corona

    NASA Technical Reports Server (NTRS)

    Raymond, John C.

    1993-01-01

    The X-ray-illuminated surface of the accretion disk in a low-mass X-ray Binary (LMXRB) and the X-ray-heated corona above the disk produce optical, UV, and soft X-ray emission lines. This paper presents 1D models of the emission line spectra and the vertical temperature and density structures at different radii. The models include a detailed treatment of the important atomic processes and an escape probability treatment of radiative transfer. Soker and Raymond (1993) use the density structure predicted by these models for a 2D Monte Carlo simulation of the photon scattering in the accretion disk corona (ADC) to examine the effects of the ADC on the angular distribution of X-rays and the flux of X-rays incident on the outer disk. This paper concentrates on the emission line fluxes for various elemental abundances and disk parameters. The UV lines of the classic LMXRBs are consistent with the model predictions. Some CNO processing is necessary to account for the nitrogen and helium abundances in Sco X-1 and other LMXRBs. Comparison of the models with observed spectra also points to a soft X-ray component with luminosity comparable to the hard X-rays. The models predict a substantial luminosity in the group of highly ionized iron lines near 100 A.

  9. The power of relativistic jets is larger than the luminosity of their accretion disks.

    PubMed

    Ghisellini, G; Tavecchio, F; Maraschi, L; Celotti, A; Sbarrato, T

    2014-11-20

    Theoretical models for the production of relativistic jets from active galactic nuclei predict that jet power arises from the spin and mass of the central supermassive black hole, as well as from the magnetic field near the event horizon. The physical mechanism underlying the contribution from the magnetic field is the torque exerted on the rotating black hole by the field amplified by the accreting material. If the squared magnetic field is proportional to the accretion rate, then there will be a correlation between jet power and accretion luminosity. There is evidence for such a correlation, but inadequate knowledge of the accretion luminosity of the limited and inhomogeneous samples used prevented a firm conclusion. Here we report an analysis of archival observations of a sample of blazars (quasars whose jets point towards Earth) that overcomes previous limitations. We find a clear correlation between jet power, as measured through the γ-ray luminosity, and accretion luminosity, as measured by the broad emission lines, with the jet power dominating the disk luminosity, in agreement with numerical simulations. This implies that the magnetic field threading the black hole horizon reaches the maximum value sustainable by the accreting matter.

  10. The power of relativistic jets is larger than the luminosity of their accretion disks.

    PubMed

    Ghisellini, G; Tavecchio, F; Maraschi, L; Celotti, A; Sbarrato, T

    2014-11-20

    Theoretical models for the production of relativistic jets from active galactic nuclei predict that jet power arises from the spin and mass of the central supermassive black hole, as well as from the magnetic field near the event horizon. The physical mechanism underlying the contribution from the magnetic field is the torque exerted on the rotating black hole by the field amplified by the accreting material. If the squared magnetic field is proportional to the accretion rate, then there will be a correlation between jet power and accretion luminosity. There is evidence for such a correlation, but inadequate knowledge of the accretion luminosity of the limited and inhomogeneous samples used prevented a firm conclusion. Here we report an analysis of archival observations of a sample of blazars (quasars whose jets point towards Earth) that overcomes previous limitations. We find a clear correlation between jet power, as measured through the γ-ray luminosity, and accretion luminosity, as measured by the broad emission lines, with the jet power dominating the disk luminosity, in agreement with numerical simulations. This implies that the magnetic field threading the black hole horizon reaches the maximum value sustainable by the accreting matter. PMID:25409827

  11. Thermal instability accretion disk model for the X-ray transient A0620-00

    NASA Technical Reports Server (NTRS)

    Huang, Min; Wheeler, J. Craig

    1989-01-01

    The limit-cycle thermal instability model for accretion disks is used to study the soft X-ray transient A0620-00. Thermal instability in geometrically thin, Keplerian alpha-model disks is reviewed. The observational constraints on A0620-00 are presented and the parameters chosen for the model are discussed. It is found that, with the adopted parameters, the model requires a central object mass of about 7 solar masses to fit the burst recurrence time. This is consistent with a black hole as the central object. The results suggest that a mass transfer instability may be responsible for outbursts.

  12. Bipolar flows, molecular gas disks, and the collapse and accretion of rotating interstellar clouds

    NASA Technical Reports Server (NTRS)

    Boss, Alan P.

    1987-01-01

    Rigorous numerical models of the collapse and accretion of rotating, axisymmetric, isothermal interstellar clouds are studied. The results show that molecular gas disks and evacuated bipolar cavities both appear to be natural consequences of the collapse of rotating interstellar clouds. Dynamically significant magnetic fields may not be necessary for explaining either phenomenon. The models strongly support theoretical models of the type where an isotropic wind from a pre-main sequence star is extrinsically collimated by a rotationally derived molecular gas cloud. The models imply that collimation should be strongest on small scales where rotational effects are most important, i.e., in the dense region of the molecular gas disk.

  13. X-ray Reflected Spectra from Accretion Disk Models. I. Constant Density Atmospheres

    NASA Technical Reports Server (NTRS)

    Garcia, Javier; Kallman, Timothy R.

    2009-01-01

    We present new models for illuminated accretion disks, their structure and reprocessed emission. We consider the effects of incident X-rays on the surface of an accretion disk by solving simultaneously the equations of radiative transfer, energy balance and ionization equilibrium over a large range of column densities. We assume plane-parallel geometry and azimuthal symmetry, such that each calculation corresponds to a ring at a given distance from the central object. Our models include recent and complete atomic data for K-shell of the iron and oxygen isonuclear sequences. We examine the effect on the spectrum of fluorescent Ka line emission and absorption in the emitted spectrum. We also explore the dependence of the spectrum on the strength of the incident X-rays and other input parameters, and discuss the importance of Comptonization on the emitted spectrum.

  14. Cepheus OB2: Disk Evolution and Accretion at 3-10 Myr

    NASA Technical Reports Server (NTRS)

    Sicilia-Aguilar, A.

    2005-01-01

    We presented the results of MMT observations of young stars for our study of protoplanetary disks at ages 1-10 Myr in Tr 37 and NGC 7160. Using low-resolution optical spectra from the Hectospec multifiber spectrograph, we have tripled the number of known low-mass cluster members, identifying 130 new members in Tr 37 and 30 in NGC 7160. We used indicators of youth to identify and classify the low-mass cluster members. With the extended samples, we estimated the disk fraction in the clusters, finding that 40% of the low-mass stars in Tr 37 are actively accreting, whereas only 1 of the 55 low-mass stars in NGC 7160 shows indications of accretion. Optical photometry and theoretical isochrones are used to refine the ages of the clusters, confirming the estimates of 4 Myr for Tr 37 and 10 Myr for NGC 7160.

  15. Variations in the accretion rate and luminosity in gravitationally unstable protostellar disks

    NASA Astrophysics Data System (ADS)

    Elbakyan, V. G.; Vorobyov, E. I.; Glebova, G. M.

    2016-10-01

    Self-consistent modeling of a protostar and protostellar disk is carried out for early stages of their evolution. The accretion rate at distances of sevral astronomical units from the protostar is appreciably variable, which is reflected in the protostar's luminosity. The amplitude of the variations in the accretion rate and luminosity grows together with the sampling period, as a consequence of the nature of gravitationally unstable protostellar disks. A comparison of model luminosity variations with those derived from observations of nearby sites of star formation shows that the model variations are appreciably lower than the observed values for sampling periods of less than 10 years, indicating the presence of additional sources of variability on small dynamical distances from the protostar.

  16. An upper limit to the electrodynamic power output from a thin accretion disk around a black hole

    NASA Technical Reports Server (NTRS)

    Park, Seok Jae; Vishniac, Ethan T.

    1994-01-01

    We examine the electrodynamic power output from a thin accretion disk surrounding a Kerr black hole. We derive an upper limit for the total power. We find that this limit is only moderately sensitive to most of the relativistic correction factors. In fact, this limit is less sensitive to the change in the black hole spin than one would expect from the change in the inner radius of the accretion disk. The disk around a rapidly rotating black hole will produce more nonthermal radiation than a similar disk around a slowly rotating black hole of identical mass, but the difference is only a factor of approximately 1.7.

  17. Convection causes enhanced magnetic turbulence in accretion disks in outburst

    SciTech Connect

    Hirose, Shigenobu; Blaes, Omer; Coleman, Matthew S. B.; Krolik, Julian H.; Sano, Takayoshi

    2014-05-20

    We present the results of local, vertically stratified, radiation magnetohydrodynamic (MHD) shearing box simulations of magneto-rotational instability (MRI) turbulence appropriate for the hydrogen ionizing regime of dwarf nova and soft X-ray transient outbursts. We incorporate the frequency-integrated opacities and equation of state for this regime, but neglect non-ideal MHD effects and surface irradiation, and do not impose net vertical magnetic flux. We find two stable thermal equilibrium tracks in the effective temperature versus surface mass density plane, in qualitative agreement with the S-curve picture of the standard disk instability model. We find that the large opacity at temperatures near 10{sup 4} K, a corollary of the hydrogen ionization transition, triggers strong, intermittent thermal convection on the upper stable branch. This convection strengthens the magnetic turbulent dynamo and greatly enhances the time-averaged value of the stress to thermal pressure ratio α, possibly by generating vertical magnetic field that may seed the axisymmetric MRI, and by increasing cooling so that the pressure does not rise in proportion to the turbulent dissipation. These enhanced stress to pressure ratios may alleviate the order of magnitude discrepancy between the α-values observationally inferred in the outburst state and those that have been measured from previous local numerical simulations of magnetorotational turbulence that lack net vertical magnetic flux.

  18. Cygnus X-1: A Case for a Magnetic Accretion Disk?

    NASA Technical Reports Server (NTRS)

    Nowak, Michael A.; Vaughan, B. A.; Dove, J.; Wilms, J.

    1996-01-01

    With the advent of Rossi X-ray Timing Explorer (RXTE), which is capable of broad spectral coverage and fast timing, as well as other instruments which are increasingly being used in multi-wavelength campaigns (via both space-based and ground-based observations), we must demand more of our theoretical models. No current model mimics all facets of a system as complex as an x-ray binary. However, a modern theory should qualitatively reproduce - or at the very least not fundamentally disagree with - all of Cygnus X-l's most basic average properties: energy spectrum (viewed within a broader framework of black hole candidate spectral behavior), power spectrum (PSD), and time delays and coherence between variability in different energy bands. Below we discuss each of these basic properties in turn, and we assess the health of one of the currently popular theories: Comptonization of photons from a cold disk. We find that the data pose substantial challenges for this theory, as well as all other in currently discussed models.

  19. Equilibrium configuration and stability of a stratus floating above accretion disks

    NASA Astrophysics Data System (ADS)

    Nakai, Takuya; Fukue, Jun

    2016-04-01

    We examine the equilibrium configurations of a stratus floating above an accretion disk, using the radiative force from the luminous disk just below the stratus. For various disk luminosities and optical depths of the stratus, the stratus can stably float on the outer disk, while a stable configuration does not exist on the inner disk. When the disk luminosity normalized by the Eddington luminosity is unity, and the stratus optical depth is around unity, the stable configuration disappears at r ≲ 50rg, rg being the Schwarzschild radius, and the stratus would be blown off as a cloudy wind, which consists of many strati with appropriate conditions. In the outer region of r ≳ 50rg, on the other hand, we find that the stable floating height is z ˜ 20rg, which is approximately two times larger than in the case of the particle. This difference is due to the anisotropic scattering effect; the stratus can get twice the momentum from radiation than it can in the particle case. The present results, that the radiation-driven cloudy wind can be easily blown off from the luminous disk, can explain observed outflows in broad absorption line quasars and ultra-fast outflow objects.

  20. ACCRETION DISK WARPING BY RESONANT RELAXATION: THE CASE OF MASER DISK NGC 4258

    SciTech Connect

    Bregman, Michal; Alexander, Tal

    2009-08-01

    The maser disk around the massive black hole (MBH) in active galaxy NGC 4258 exhibits an O(10 deg.) warp on the O(0.1 pc) scale. The physics driving the warp is still debated. Suggested mechanisms include torquing by relativistic frame dragging or by radiation pressure. We propose here a new warping mechanism: resonant torquing of the disk by stars in the dense cusp around the MBH. We show that resonant torquing can induce such a warp over a wide range of observed and deduced physical parameters of the maser disk.

  1. DEPLETION OF MOLECULAR GAS BY AN ACCRETION OUTBURST IN A PROTOPLANETARY DISK

    SciTech Connect

    Banzatti, A.; Pontoppidan, K. M.; Muzerolle, J.; Bruderer, S.; Meyer, M. R.

    2015-01-01

    We investigate new and archival 3-5 μm high-resolution (∼3 km s{sup –1}) spectroscopy of molecular gas in the inner disk of the young solar-mass star EX Lupi, taken during and after the strong accretion outburst of 2008. The data were obtained using the CRIRES spectrometer at the European Southern Observatory Very Large Telescope in 2008 and 2014. In 2008, emission lines from CO, H{sub 2}O, and OH were detected with broad profiles tracing gas near and within the corotation radius (0.02-0.3 AU). In 2014, the spectra display marked differences. The CO lines, while still detected, are much weaker, and the H{sub 2}O and OH lines have disappeared altogether. At 3 μm a veiled stellar photospheric spectrum is observed. Our analysis finds that the molecular gas mass in the inner disk has decreased by an order of magnitude since the outburst, matching a similar decrease in the accretion rate onto the star. We discuss these findings in the context of a rapid depletion of material accumulated beyond the disk corotation radius during quiescent periods, as proposed by models of episodic accretion in EXor-type young stars.

  2. REVISITING PUTATIVE COOL ACCRETION DISKS IN ULTRALUMINOUS X-RAY SOURCES

    SciTech Connect

    Miller, J. M.; King, A. L.; Reynolds, M. T.; Reis, R. C.; Walton, D. J.; Fabian, A. C.; Miller, M. C.

    2013-10-20

    Soft, potentially thermal spectral components observed in some ultra-luminous X-ray sources (ULXs) can be fit with models for emission from cool, optically thick accretion disks. If that description is correct, the low temperatures that are observed imply accretion onto 'intermediate-mass' black holes. Subsequent work has found that these components may follow an inverse relationship between luminosity and temperature, implying a non-blackbody origin for this emission. We have re-analyzed numerous XMM-Newton spectra of extreme ULXs. Crucially, observations wherein the source fell on a chip gap were excluded owing to their uncertain flux calibration, and the neutral column density along the line of sight to a given source was jointly determined by multiple spectra. The luminosity of the soft component is found to be positively correlated with temperature, and to be broadly consistent with L∝T {sup 4} in the measured band pass, as per blackbody emission from a standard thin disk. These results are nominally consistent with accretion onto black holes with masses above the range currently known in Galactic X-ray binaries, though there are important caveats. Emission from inhomogeneous or super-Eddington disks may also be consistent with the data.

  3. Depletion of Molecular Gas by an Accretion Outburst in a Protoplanetary Disk

    NASA Astrophysics Data System (ADS)

    Banzatti, A.; Pontoppidan, K. M.; Bruderer, S.; Muzerolle, J.; Meyer, M. R.

    2015-01-01

    We investigate new and archival 3-5 μm high-resolution (~3 km s-1) spectroscopy of molecular gas in the inner disk of the young solar-mass star EX Lupi, taken during and after the strong accretion outburst of 2008. The data were obtained using the CRIRES spectrometer at the European Southern Observatory Very Large Telescope in 2008 and 2014. In 2008, emission lines from CO, H2O, and OH were detected with broad profiles tracing gas near and within the corotation radius (0.02-0.3 AU). In 2014, the spectra display marked differences. The CO lines, while still detected, are much weaker, and the H2O and OH lines have disappeared altogether. At 3 μm a veiled stellar photospheric spectrum is observed. Our analysis finds that the molecular gas mass in the inner disk has decreased by an order of magnitude since the outburst, matching a similar decrease in the accretion rate onto the star. We discuss these findings in the context of a rapid depletion of material accumulated beyond the disk corotation radius during quiescent periods, as proposed by models of episodic accretion in EXor-type young stars.

  4. X-ray Light Curves and Accretion Disk Structure of EX Hydrae

    SciTech Connect

    Hoogerwerf, R; Brickhouse, N S; Mauche, C W

    2005-04-12

    We present X-ray light curves for the cataclysmic variable EX Hydrae obtained with the Chandra High Energy Transmission Grating Spectrometer and the Extreme Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the shape and amplitude of the binary light curve and discuss a new feature: the phase of the minimum in the binary light curve, associated with absorption by the bulge on the accretion disk, increases with wavelength. We discuss several scenarios that could account for this trend and conclude that, most likely, the ionization state of the bulge gas is not constant, but rather decreases with binary phase. We also conclude that photoionization of the bulge by radiation originating from the white dwarf is not the main source of ionization, but that it is heated by shocks originating from the interaction between the in-flowing material from the companion and the accretion disk. The findings in this paper provide a strong test for accretion disk models in close binary systems.

  5. Accretion Disk Dynamo as the Trigger for X-Ray Binary State Transitions

    NASA Astrophysics Data System (ADS)

    Begelman, Mitchell C.; Armitage, Philip J.; Reynolds, Christopher S.

    2015-08-01

    Magnetohydrodynamic accretion disk simulations suggest that much of the energy liberated by the magnetorotational instability (MRI) can be channeled into large-scale toroidal magnetic fields through dynamo action. Under certain conditions, this field can dominate over gas and radiation pressure in providing vertical support against gravity, even close to the midplane. Using a simple model for the creation of this field, its buoyant rise, and its coupling to the gas, we show how disks could be driven into this magnetically dominated state and deduce the resulting vertical pressure and density profiles. Applying an established criterion for MRI to operate in the presence of a toroidal field, we show that magnetically supported disks can have two distinct MRI-active regions, separated by a “dead zone” where local MRI is suppressed, but where magnetic energy continues to flow upward from the dynamo region below. We suggest that the relative strengths of the MRI zones, and the local poloidal flux, determine the spectral states of X-ray binaries. Specifically, “intermediate” and “hard” accretion states occur when MRI is triggered in the hot, upper zone of the corona, while disks in “soft” states do not develop the upper MRI zone. We discuss the conditions under which various transitions should take place and speculate on the relationship of dynamo activity to the various types of quasi-periodic oscillations that sometimes appear in the hard spectral components. The model also explains why luminous accretion disks in the “soft” state show no signs of the thermal/viscous instability predicted by standard α-models.

  6. ACCRETION DISK DYNAMO AS THE TRIGGER FOR X-RAY BINARY STATE TRANSITIONS

    SciTech Connect

    Begelman, Mitchell C.; Armitage, Philip J.; Reynolds, Christopher S.

    2015-08-20

    Magnetohydrodynamic accretion disk simulations suggest that much of the energy liberated by the magnetorotational instability (MRI) can be channeled into large-scale toroidal magnetic fields through dynamo action. Under certain conditions, this field can dominate over gas and radiation pressure in providing vertical support against gravity, even close to the midplane. Using a simple model for the creation of this field, its buoyant rise, and its coupling to the gas, we show how disks could be driven into this magnetically dominated state and deduce the resulting vertical pressure and density profiles. Applying an established criterion for MRI to operate in the presence of a toroidal field, we show that magnetically supported disks can have two distinct MRI-active regions, separated by a “dead zone” where local MRI is suppressed, but where magnetic energy continues to flow upward from the dynamo region below. We suggest that the relative strengths of the MRI zones, and the local poloidal flux, determine the spectral states of X-ray binaries. Specifically, “intermediate” and “hard” accretion states occur when MRI is triggered in the hot, upper zone of the corona, while disks in “soft” states do not develop the upper MRI zone. We discuss the conditions under which various transitions should take place and speculate on the relationship of dynamo activity to the various types of quasi-periodic oscillations that sometimes appear in the hard spectral components. The model also explains why luminous accretion disks in the “soft” state show no signs of the thermal/viscous instability predicted by standard α-models.

  7. Can accretion disk properties observationally distinguish black holes from naked singularities?

    NASA Astrophysics Data System (ADS)

    Kovács, Z.; Harko, T.

    2010-12-01

    Naked singularities are hypothetical astrophysical objects, characterized by a gravitational singularity without an event horizon. Penrose has proposed a conjecture, according to which there exists a cosmic censor who forbids the occurrence of naked singularities. Distinguishing between astrophysical black holes and naked singularities is a major challenge for present day observational astronomy. In the context of stationary and axially symmetrical geometries, a possibility of differentiating naked singularities from black holes is through the comparative study of thin accretion disks properties around rotating naked singularities and Kerr-type black holes, respectively. In the present paper, we consider accretion disks around axially-symmetric rotating naked singularities, obtained as solutions of the field equations in the Einstein-massless scalar field theory. A first major difference between rotating naked singularities and Kerr black holes is in the frame dragging effect, the angular velocity of a rotating naked singularity being inversely proportional to its spin parameter. Because of the differences in the exterior geometry, the thermodynamic and electromagnetic properties of the disks (energy flux, temperature distribution and equilibrium radiation spectrum) are different for these two classes of compact objects, consequently giving clear observational signatures that could discriminate between black holes and naked singularities. For specific values of the spin parameter and of the scalar charge, the energy flux from the disk around a rotating naked singularity can exceed by several orders of magnitude the flux from the disk of a Kerr black hole. In addition to this, it is also shown that the conversion efficiency of the accreting mass into radiation by rotating naked singularities is always higher than the conversion efficiency for black holes, i.e., naked singularities provide a much more efficient mechanism for converting mass into radiation than black

  8. C/O and Snowline Locations in Protoplanetary Disks: The Effect of Radial Drift and Viscous Gas Accretion

    NASA Astrophysics Data System (ADS)

    Piso, Ana-Maria A.; Öberg, Karin I.; Birnstiel, Tilman; Murray-Clay, Ruth A.

    2015-12-01

    The C/O ratio is a defining feature of both gas giant atmospheric and protoplanetary disk chemistry. In disks, the C/O ratio is regulated by the presence of snowlines of major volatiles at different distances from the central star. We explore the effect of the radial drift of solids and viscous gas accretion onto the central star on the snowline locations of the main C and O carriers in a protoplanetary disk, H2O, CO2, and CO, and their consequences for the C/O ratio in gas and dust throughout the disk. We determine the snowline locations for a range of fixed initial particle sizes and disk types. For our fiducial disk model, we find that grains with sizes ∼0.5 cm ≲ s ≲ 7 m for an irradiated disk and ∼0.001 cm ≲ s ≲ 7 m for an evolving and viscous disk desorb at a size-dependent location in the disk, which is independent of the particle's initial position. The snowline radius decreases for larger particles, up to sizes of ∼7 m. Compared to a static disk, we find that radial drift and gas accretion in a viscous disk move the H2O snowline inwards by up to 40%, the CO2 snowline by up to 60%, and the CO snowline by up to 50%. We thus determine an inner limit on the snowline locations when radial drift and gas accretion are accounted for.

  9. Generation of Magnetic Field on the Accretion Disk around a Proto-first-star

    NASA Astrophysics Data System (ADS)

    Shiromoto, Yuki; Susa, Hajime; Hosokawa, Takashi

    2014-02-01

    The generation process of a magnetic field around a proto-first-star is studied. Utilizing the recent numerical results of proto-first-star formation based on radiation hydrodynamics simulations, we assess the magnetic field strength generated by the radiative force and the Biermann battery effect. We find that a magnetic field of ~10-9 G is generated on the surface of the accretion disk around the proto-first-star. The field strength on the accretion disk is smaller by two orders of magnitude than the critical value, above which the gravitational fragmentation of the disk is suppressed. Thus, the generated seed magnetic field hardly affect the dynamics of on-site first star formation directly, unless an efficient amplification process is taken into consideration. We also find that the generated magnetic field is continuously blown out from the disk on the outflows to the poles, that are driven by the thermal pressure of photoheated gas. The strength of the diffused magnetic field in low-density regions is ~10-14-10-13 G at n H = 103 cm-3, which could play an important role in the next generation star formation, as well as the seeds of the magnetic field in the present-day universe.

  10. Generation of magnetic field on the accretion disk around a proto-first-star

    SciTech Connect

    Shiromoto, Yuki; Susa, Hajime; Hosokawa, Takashi

    2014-02-20

    The generation process of a magnetic field around a proto-first-star is studied. Utilizing the recent numerical results of proto-first-star formation based on radiation hydrodynamics simulations, we assess the magnetic field strength generated by the radiative force and the Biermann battery effect. We find that a magnetic field of ∼10{sup –9} G is generated on the surface of the accretion disk around the proto-first-star. The field strength on the accretion disk is smaller by two orders of magnitude than the critical value, above which the gravitational fragmentation of the disk is suppressed. Thus, the generated seed magnetic field hardly affect the dynamics of on-site first star formation directly, unless an efficient amplification process is taken into consideration. We also find that the generated magnetic field is continuously blown out from the disk on the outflows to the poles, that are driven by the thermal pressure of photoheated gas. The strength of the diffused magnetic field in low-density regions is ∼10{sup –14}-10{sup –13} G at n {sub H} = 10{sup 3} cm{sup –3}, which could play an important role in the next generation star formation, as well as the seeds of the magnetic field in the present-day universe.

  11. ANGULAR MOMENTUM TRANSPORT AND VARIABILITY IN BOUNDARY LAYERS OF ACCRETION DISKS DRIVEN BY GLOBAL ACOUSTIC MODES

    SciTech Connect

    Belyaev, Mikhail A.; Stone, James M.; Rafikov, Roman R.

    2012-11-20

    Disk accretion onto a weakly magnetized central object, e.g., a star, is inevitably accompanied by the formation of a boundary layer near the surface, in which matter slows down from the highly supersonic orbital velocity of the disk to the rotational velocity of the star. We perform high-resolution two-dimensional hydrodynamical simulations in the equatorial plane of an astrophysical boundary layer with the goal of exploring the dynamics of non-axisymmetric structures that form there. We generically find that the supersonic shear in the boundary layer excites non-axisymmetric quasi-stationary acoustic modes that are trapped between the surface of the star and a Lindblad resonance in the disk. These modes rotate in a prograde fashion, are stable for hundreds of orbital periods, and have a pattern speed that is less than and of the order of the rotational velocity at the inner edge of the disk. The origin of these intrinsically global modes is intimately related to the operation of a corotation amplifier in the system. Dissipation of acoustic modes in weak shocks provides a universal mechanism for angular momentum and mass transport even in purely hydrodynamic (i.e., non-magnetized) boundary layers. We discuss the possible implications of these trapped modes for explaining the variability seen in accreting compact objects.

  12. Large-Scale Magnetic Field in Accretion Disks and Relativistic Poynting-Flux Jets

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Bisnovatyi-Kogan, G. S.

    In earlier works we pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because of hydrodynamic and/or magnetorotational (MRI) instabilities are suppressed high in the disk where the magnetic and radiation pressures are larger than the plasma thermal pressure. We have derived equations for the vertical profiles of stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity and the fact that the turbulence vanishes at the surface of the disk. Our recent analysis in Ref. 1 shows that the inward or outward advection of the large-scale magnetic field depends on the ratio {R} of the accretion power going into magnetic disk winds to the viscous power dissipation and the plasma-β which is the ratio of the midplane plasma pressure to the magnetic pressure. Recent radio emission, polarization, and Faraday rotation maps of the radio jet of the galaxy 3C303 have been obtained in Ref. 2 and show that one component of this jet has a galactic-scale electric current of 3 × 1018 A flowing along the jet axis. We show that this current can be used to calculate the electromagnetic energy flow in this magnetically dominated jet.

  13. Gamma-ray bursts from stellar mass accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Woosley, S. E.

    1993-01-01

    A cosmological model for gamma-ray bursts is explored in which the radiation is produced as a broadly beamed pair fireball along the rotation axis of an accreting black hole. The black hole may be a consequence of neutron star merger or neutron star-black hole merger, but for long complex bursts, it is more likely to come from the collapse of a single Wolf-Rayet star endowed with rotation ('failed' Type Ib supernova). The disk is geometrically thick and typically has a mass inside 100 km of several tenths of a solar mass. In the failed supernova case, the disk is fed for a longer period of time by the collapsing star. At its inner edge the disk is thick to its own neutrino emission and evolves on a viscous time scale of several seconds. In a region roughly 30 km across, interior to the accretion disk and along its axis of rotation, a pair fireball is generated by neutrino annihilation and electron-neutrino scattering which deposit approximately 10 exp 50 ergs/s.

  14. THE EFFECT OF A TIME-VARYING ACCRETION DISK SIZE ON QUASAR MICROLENSING LIGHT CURVES

    SciTech Connect

    Blackburne, Jeffrey A.; Kochanek, Christopher S. E-mail: ckochanek@astronomy.ohio-state.ed

    2010-08-01

    Microlensing perturbations to the magnification of gravitationally lensed quasar images are dependent on the angular size of the quasar. If quasar variability at visible wavelengths is caused by a change in the area of the accretion disk, it will affect the microlensing magnification. We derive the expected signal, assuming that the luminosity scales with some power of the disk area, and estimate its amplitude using simulations. We discuss the prospects for detecting the effect in real-world data and for using it to estimate the logarithmic slope of the luminosity's dependence on disk area. Such an estimate would provide a direct test of the standard thin accretion disk model. We tried fitting six seasons of the light curves of the lensed quasar HE 0435-1223 including this effect as a modification to the Kochanek et al. approach to estimating time delays. We find a dramatic improvement in the goodness of fit and relatively plausible parameters, but a robust estimate will require a full numerical calculation in order to correctly model the strong correlations between the structure of the microlensing magnification patterns and the magnitude of the effect. We also comment briefly on the effect of this phenomenon for the stability of time-delay estimates.

  15. Spectral eclipse mapping of the accretion disk in the nova-like variable UX Ursae Majoris

    NASA Technical Reports Server (NTRS)

    Rutten, R. G. M.; Dhillon, V. S.; Horne, K.; Kuulkers, E.

    1994-01-01

    We analyze narrow-band eclipse light curves of the nova-like cataclysmic variable UX UMa, obtained from low-resolution spectra spanning lambda lambda 3600-9800 A . The light curves for narrow bands in the continuum as well as those for individual spectral lines are treated independently, and are used to construct images of the accretion disk's brightness distribution using the maximum-entropy eclipse-mapping technique. Particular attention is paid to the propagation of statistical uncertainties in the data and to how the analysis may introduce systematic errors in the final result. From the many narrrow band images we have reconstructed the spectra from isolated parts of the accretion disk. These spectra reveal that the inner disk radiates a continuum spectrum which peaks in the near UV and has the hydrogen Balmer lines in absorption (with the exception of H-alpha), whereas the outer disk is much fainter, has a much redder spectrum, and has Balmer emission lines. Our analysis reveals the presence of an uneclipsed component of the total light, whose spectrum is very red and has Balmer lines in emission. This unexpected feature of the eclipse mapping technique offers a new tool for an independent assessment of the secondary star's spectrum in eclipsing cataclysmic variables.

  16. Ion Viscosity Mediated by Tangled Magnetic Fields: An Application to Black Hole Accretion Disks

    NASA Technical Reports Server (NTRS)

    Subramanian, Prasad; Becker, Peter A.; Kafatos, Menas

    1996-01-01

    We examine the viscosity associated with the shear stress exerted by ions in the presence of a tangled magnetic field. As an application, we consider the effect of this mechanism on the structure of black hole accretion disks. We do not attempt to include a self-consistent description of the magnetic field. Instead, we assume the existence of a tangled field with coherence length lambda(sub coh), which is the average distance between the magnetic 'kinks' that scatter the particles. For simplicity, we assume that the field is self-similar, and take lambda(sub coh) to be a fixed fraction zeta of the local disk height H. Ion viscosity in the presence of magnetic fields is generally taken to be the cross-field viscosity, wherein the effective mean free path is the ion Larmor radius lambda(sub L), which is much less than the ion-ion Coulomb mean free path A(sub ii) in hot accretion disks. However, we arrive at a formulation for a 'hybrid' viscosity in which the tangled magnetic field acts as an intermediary in the transfer of momentum between different layers in the shear flow. The hybrid viscosity greatly exceeds the standard cross-field viscosity when (lambda/lambda(sub L)) much greater than (lambda(sub L)/lambda(sub ii)), where lambda = ((lambda(sub ii)(sup -1) + lambda(sub (coh)(sup -1))(sup -1) is the effective mean free path for the ions. This inequality is well satisfied in hot accretion disks, which suggests that the ions may play a much larger role in the momentum transfer process in the presence of magnetic fields than was previously thought. The effect of the hybrid viscosity on the structure of a steady-state, two-temperature, quasi-Keplerian accretion disk is analyzed. The hybrid viscosity is influenced by the degree to which the magnetic field is tangled (represented by zeta = lambda(sub coh)), and also by the relative accretion rate M/M(sub E), where M(sub E) = L(sub E)/c(sup 2) and L(sub E) is the Eddington luminosity. We find that ion viscosity in the

  17. Stratified magnetically driven accretion-disk winds and their relations to jets

    SciTech Connect

    Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Contopoulos, Ioannis

    2014-01-10

    We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly ionized ultra-fast outflows (UFOs) in active galactic nuclei (AGNs), in a single unifying approach. We present the density n(r, θ), ionization parameter ξ(r, θ), and velocity structure v(r, θ) of such ionized winds for typical values of their fluid-to-magnetic flux ratio, F, and specific angular momentum, H, for which wind solutions become super-Alfvénic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller H show a poloidal geometry of narrower opening angles with their Alfvén surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, r, and distinct values of n, ξ, and v consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.

  18. Stratified Magnetically Driven Accretion-Disk Winds and Their Relations To Jets

    NASA Technical Reports Server (NTRS)

    Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Behar, Ehud; Contopoulos, Ioannis

    2013-01-01

    We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly ionized ultra-fast outflows (UFOs) in active galactic nuclei (AGNs), in a single unifying approach. We present the density n(r, theta), ionization parameter xi(r, theta), and velocity structure v(r, theta) of such ionized winds for typical values of their fluid-to-magnetic flux ratio, F, and specific angular momentum, H, for which wind solutions become super-Alfvenic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller H show a poloidal geometry of narrower opening angles with their Alfv´en surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, r, and distinct values of n, xi, and v consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.

  19. Stratified Magnetically Driven Accretion-disk Winds and Their Relations to Jets

    NASA Astrophysics Data System (ADS)

    Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Behar, Ehud; Contopoulos, Ioannis

    2014-01-01

    We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly ionized ultra-fast outflows (UFOs) in active galactic nuclei (AGNs), in a single unifying approach. We present the density n(r, θ), ionization parameter ξ(r, θ), and velocity structure v(r, θ) of such ionized winds for typical values of their fluid-to-magnetic flux ratio, F, and specific angular momentum, H, for which wind solutions become super-Alfvénic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller H show a poloidal geometry of narrower opening angles with their Alfvén surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, r, and distinct values of n, ξ, and v consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.

  20. VISCOUS ACCRETION OF A POLYTROPIC SELF-GRAVITATING DISK IN THE PRESENCE OF WIND

    SciTech Connect

    Abbassi, Shahram; Nourbakhsh, Erfan; Shadmehri, Mohsen E-mail: e.nourbakhsh@mail.sbu.ac.ir

    2013-03-10

    Self-similar and semi-analytical solutions are found for the height-averaged equations governing the dynamical behavior of a polytropic, self-gravitating disk under the effects of winds around the nascent object. In order to describe the time evolution of the system, we adopt a radius-dependent mass loss rate, then highlight its importance on both the traditional {alpha} and innovative {beta} models of viscosity prescription. In agreement with some other studies, our solutions represent that the Toomre parameter is less than one in most regions on the {beta}-disk, which indicates that in such disks gravitational instabilities can occur at various distances from the central accretor. So, the {beta}-disk model might provide a good explanation of how the planetary systems form. The purpose of the present work is twofold: examining the structure of a disk with wind in comparison to a no-wind solution and seeing whether the adopted viscosity prescription significantly affects the dynamical behavior of the disk-wind system. We also considered the temperature distribution in our disk by a polytropic condition. The solutions imply that, under our boundary conditions, the radial velocity is larger for {alpha}-disks and increases as wind becomes stronger in both viscosity models. Also, we noticed that the disk thickness increases by amplifying the wind or adopting larger values for the polytropic exponent {gamma}. It also may globally decrease if one prescribes a {beta}-model for the viscosity. Moreover, in both viscosity models, the surface density and mass accretion rate diminish as the wind gets stronger or {gamma} increases.

  1. The transport of angular momentum by gravitational instabilities and Rossby vortices in accretion disks

    NASA Astrophysics Data System (ADS)

    Currier, Nathaniel W.

    We propose a model for the birth of spiral galaxies and the supermassive black holes (SMBHs) at their centers. It all starts when a galaxy-mass gas condensation collapses to ~ 200 × the background density. It experiences weak tidal torques from similar condensations, which establish its spin parameter l. It forms a Lyman-a (Lya) cloud, then undergoes an inviscid, angular-momentum- preserving collapse to a Mestel disk with a flat rotation curve (FRCD). A FRCD has v ~ const, M accretion disk instead of stars. We invoke two mechanisms to drive accretion: The self- gravity instability (SGI) and the Rossby vortex instability (RVI). Both mechanisms transport angular momentum coherently, so they easily dominate turbulent mechanisms wherever the disk is thin. The popular magneto-rotational instability (MRI) is semi-coherent, but it's not required for our model, so we leave it for further study. We use a 2-D Eulerian hydro code to simulate the SGI and RVI in both FRCDs and Keplerian disks. We explore the triggers of these instabilities, namely, the Toomre parameter Q in SGI-unstable FRCDs and pressure jumps in RVI-unstable Keplerian disks. We confirm that Q [Special characters omitted.] 1 triggers the SGI in FRCDs and that D P/P [Special characters omitted.] 5 generates robust Rossby vortices in Keplerian disks. We also find that these instabilities interact in the transition region between these two types of disks. We relate all this to our self-consistent model

  2. Global Time Dependent Solutions of Stochastically Driven Standard Accretion Disks: Development of Hydrodynamical Code

    NASA Astrophysics Data System (ADS)

    Wani, Naveel; Maqbool, Bari; Iqbal, Naseer; Misra, Ranjeev

    2016-07-01

    X-ray binaries and AGNs are powered by accretion discs around compact objects, where the x-rays are emitted from the inner regions and uv emission arise from the relatively cooler outer parts. There has been an increasing evidence that the variability of the x-rays in different timescales is caused by stochastic fluctuations in the accretion disc at different radii. These fluctuations although arise in the outer parts of the disc but propagate inwards to give rise to x-ray variability and hence provides a natural connection between the x-ray and uv variability. There are analytical expressions to qualitatively understand the effect of these stochastic variabilities, but quantitative predictions are only possible by a detailed hydrodynamical study of the global time dependent solution of standard accretion disc. We have developed numerical efficient code (to incorporate all these effects), which considers gas pressure dominated solutions and stochastic fluctuations with the inclusion of boundary effect of the last stable orbit.

  3. The clustering amplitude of X-ray-selected AGN at z ˜ 0.8: evidence for a negative dependence on accretion luminosity

    NASA Astrophysics Data System (ADS)

    Mountrichas, G.; Georgakakis, A.; Menzel, M.-L.; Fanidakis, N.; Merloni, A.; Liu, Z.; Salvato, M.; Nandra, K.

    2016-04-01

    The northern tile of the wide-area and shallow XMM-XXL X-ray survey field is used to estimate the average dark matter halo mass of relatively luminous X-ray-selected active galactic nucleus (AGN) [log {L}_X (2-10 keV)= 43.6^{+0.4}_{-0.4} erg s^{-1}] in the redshift interval z = 0.5-1.2. Spectroscopic follow-up observations of X-ray sources in the XMM-XXL field by the Sloan telescope are combined with the VIMOS Public Extragalactic Redshift Survey spectroscopic galaxy survey to determine the cross-correlation signal between X-ray-selected AGN (total of 318) and galaxies (about 20 000). We model the large scales (2-25 Mpc) of the correlation function to infer a mean dark matter halo mass of log M / (M_{{⊙}} h^{-1}) = 12.50 ^{+0.22} _{-0.30} for the X-ray-selected AGN sample. This measurement is about 0.5 dex lower compared to estimates in the literature of the mean dark matter halo masses of moderate-luminosity X-ray AGN [LX(2-10 keV) ≈ 1042-1043 erg s- 1] at similar redshifts. Our analysis also links the mean clustering properties of moderate-luminosity AGN with those of powerful ultraviolet/optically selected QSOs, which are typically found in haloes with masses few times 1012 M⊙. There is therefore evidence for a negative luminosity dependence of the AGN clustering. This is consistent with suggestions that AGN have a broad dark matter halo mass distribution with a high mass tail that becomes subdominant at high accretion luminosities. We further show that our results are in qualitative agreement with semi-analytic models of galaxy and AGN evolution, which attribute the wide range of dark matter halo masses among the AGN population to different triggering mechanisms and/or black hole fuelling modes.

  4. Sustained Accretion on Gas Giants Surrounded by Low-Turbulence Circumplanetary Disks

    NASA Astrophysics Data System (ADS)

    D'Angelo, Gennaro; Marzari, Francesco

    2015-11-01

    Gas giants more massive than Saturn acquire most of their envelope while surrounded by a circumplanetary disk (CPD), which extends over a fraction of the planet’s Hill radius. Akin to circumstellar disks, CPDs may be subject to MRI-driven turbulence and contain low-turbulence regions, i.e., dead zones. It was suggested that CPDs may inhibit sustained gas accretion, thus limiting planet growth, because gas transport through a CPD may be severely reduced by a dead zone, a consequence at odds with the presence of Jupiter-mass (and larger) planets. We studied how an extended dead zone influences gas accretion on a Jupiter-mass planet, using global 3D hydrodynamics calculations with mesh refinements. The accretion flow from the circumstellar disk to the CPD is resolved locally at the length scale Rj, Jupiter's radius. The gas kinematic viscosity is assumed to be constant and the dead zone around the planet is modeled as a region of much lower viscosity, extending from ~Rj out to ~60Rj and off the mid-plane for a few CPD scale heights. We obtain accretion rates only marginally smaller than those reported by, e.g., D'Angelo et al. (2003), Bate et al. (2003), Bodenheimer et al. (2013), who applied the same constant kinematic viscosity everywhere, including in the CPD. As found by several previous studies (e.g., D’Angelo et al. 2003; Bate et al. 2003; Tanigawa et al. 2012; Ayliffe and Bate 2012; Gressel et al. 2013; Szulágyi et al. 2014), the accretion flow does not proceed through the CPD mid-plane but rather at and above the CPD surface, hence involving MRI-active regions (Turner et al. 2014). We conclude that the presence of a dead zone in a CPD does not inhibit gas accretion on a giant planet. Sustained accretion in the presence of a CPD is consistent not only with the formation of Jupiter but also with observed extrasolar planets more massive than Jupiter. We place these results in the context of the growth and migration of a pair of giant planets locked in the 2

  5. Probing the connection between the accretion disk, outflows and the jet in 3C111

    NASA Astrophysics Data System (ADS)

    Tombesi, Francesco

    2011-10-01

    Recent XMM-Newton and Suzaku observations of 3C111 demonstrated the presence of ultra-fast outflows (UFOs) with v~0.1c and their relation with the accretion disk. Independent studies found that X-ray dips are followed by ejection of superluminal radio knots, therefore providing a proof of the disk-jet connection. We acquired evidence that UFOs are preferentially present between X-ray dips and new knots, possibly indicating also a link between disk outflows and the jet. The goal of this XMM-Newton proposal is to confirm this evidence. Given the strong correlation with X-rays, we will use an ongoing optical monitoring campaign to trigger a 90ks observation within two days of a dip to detect a UFO and we request a possible additional 60ks >15 days after to compare with the non-dipped state.

  6. A test of synthetic accretion disk spectra using ultraviolet flux distributions of novalike variables

    NASA Technical Reports Server (NTRS)

    Wade, Richard A.

    1988-01-01

    Ultraviolet (UV) fluxes and other data for a sample of nine novalike cataclysmic variables are assembled from the literature. The UV fluxes and colors are compared to spectra of steady state model accretion disks, constructed using either Planck functions or model stellar atmosphere spectra as elementary radiators. Deficiencies are found with both sets of models. The restriction to steady state radial temperature profiles is relaxed for the 'stellar atmosphere' disks, but these generalized models still fail to account simultaneously for the absolute UV flux and the UV color of the observed spectra. The conclusion is that stellar atmosphere models do not reflect the physics of disks. More appropriate models probably can and definitely should be made.

  7. Emission-Line Eclipse Mapping of Velocity Fields in a Dwarf-Nova Accretion Disk

    NASA Astrophysics Data System (ADS)

    Makita, Makoto; Mineshige, Shin

    2002-06-01

    We propose a new method, emission-line eclipse mapping, to map the velocity fields of an accretion disk in position space. Quiescent dwarf novae usually exhibit double-peaked emission-line profiles because of disk rotation. Since a part of a disk having a different line-of-sight velocity is successively obscured by a companion in eclipsing systems, they show time-varying line profiles. We calculated the time changes of the emission-line profiles, assuming Keplerian rotation fields (vφ ~ r-1/2 with r being the distance from the disk center) and an emissivity distribution of j ~ r-3/2. We, then, applied the usual eclipse mapping technique to the light curves at each of 12-24 wavelengths across the line center to map the region with the same line-of-sight velocity. The reconstructed images typically exhibit a `two-eye' pattern for high line-of-sight velocities, and we can recover the relation, vφ ~ d-1/2, on the assumption of an axisymmetric disk, where d is the separation between the two `eyes'. We will be able to probe the Keplerian rotation law, the most fundamental assumption adopted in many disk models, by high-speed spectroscopic observations with 8-m class telescopes.

  8. Radiation, Gas and Magnetic Fields: Understanding Accretion Disks with Real Physics

    NASA Astrophysics Data System (ADS)

    Tao, Ted

    2011-01-01

    This dissertation studies some of the fundamental physics ingredients that underlie the theory of astrophysical accretion disks. We begin by focusing on local radiation magnetohydrodynamic instabilities in static, optically thick, vertically stratified media with constant flux mean opacity. Our analysis includes the effects of vertical gradients in a horizontal background magnetic field. Assuming rapid radiative diffusion, we use the zero gas pressure limit as an entry point for investigating the coupling between the photon bubble instability and the Parker instability. We find that the two instabilities transition smoothly into each other at a characteristic wavelength that is approximately equal to the magnetic pressure scale height times the ratio of radiation to magnetic pressure gradient forces. The Parker instability exists for longer wavelengths, while photon bubbles exist for wavelengths shorter than the transition wavelength. We also consider the effects of finite gas pressure on the coupled instabilities. Finite gas pressure introduces an additional short wavelength limit to the Parker-like behavior, and also limits the growth rate of the photon bubble instability to a constant value at high wave numbers. Finally, our analytic infinite wavenumber perturbation calculation strongly suggest that magnetic pressure gradients do not modify the photon bubble growth rate in the asymptotic regime. Our results may explain why photon bubbles have not yet been observed in recent stratified shearing box accretion disk simulations. Photon bubbles may physically exist in simulations with high radiation to gas pressure ratios, but higher spatial resolution will be needed to resolve the asymptotically growing unstable wavelengths. Next, we turn to the effects of local dissipation physics on the spectra and vertical structure of high luminosity stellar mass black hole X-ray binary accretion disks. More specifically, we present spectral calculations of non-LTE accretion

  9. 1H 1752 + 081: an eclipsing cataclysmic variable with a small accretion disk

    NASA Astrophysics Data System (ADS)

    Silber, Andrew D.; Remillard, Ronald A.; Horne, Keith; Bradt, Hale V.

    1994-04-01

    We announce the discovery of an eclipsing nova-like cataclysmic variable (CV) as the optical counterpart to the HEAO 1 X-ray source 1H1752 + 081. This CV has an orbital period of 1.882801 hr, a high equivalent width of H-beta, and an average mv of 16.4 out of the eclipse. A geometric model is constructed from observations of the eclipse ingress and egress in many optical bandpasses. The broad-band emission originates primarily in two regions; the disk/accretion stream 'hot spot' and a compact central component, which may be a spot on the white dwarf surface, the entire white dwarf surface or the boundary layer between the accretion disk and the white dwarf surface. Based on the durations and offsets of the two eclipses we determined the mass ratio q = 2.5 +/- 0.6 and the angle of inclination i = 77 deg +/- 2 deg. If the central component is the entire white dwarf surface the masses of the stars are M1 = 0.80 +/- 0.06 solar masses and M2 = 0.32 +/- 0.06 solar masses. The disk is faint and small (RD = 0.25 +/- 0.05 rL1, where rL1 is the distance from the primary to the L1 point), compared to other eclipsing CVs. The small disk may result from the removal of angular momentum from the accretion disk by the magnetic field of the white dwarf; this CV may be a DQ Her type with a slowly rotating white dwarf. The emission-line velocities do not show the 'Z-wave' expected from the eclipse of a Keplerian accretion disk, nor do they have the correct phasing to originate near the white dwarf. The most likely origin of the line emission is the hot spot. The secondary star is visible at wavelengths greater than or equal to 6000 A during eclipse. We estimate a spectral type approximately M6 which, together with the observed m1 = 16.94 during eclipse, results in a distance estimate of 150 +/- 27 pc.

  10. Improved reflection models of black hole accretion disks: Treating the angular distribution of X-rays

    SciTech Connect

    García, J.; Steiner, J. F.; McClintock, J. E.; Brenneman, L. E-mail: jsteiner@head.cfa.harvard.edu E-mail: lbrenneman@cfa.harvard.edu; and others

    2014-02-20

    X-ray reflection models are used to constrain the properties of the accretion disk, such as the degree of ionization of the gas and the elemental abundances. In combination with general relativistic ray tracing codes, additional parameters like the spin of the black hole and the inclination to the system can be determined. However, current reflection models used for such studies only provide angle-averaged solutions for the flux reflected at the surface of the disk. Moreover, the emission angle of the photons changes over the disk due to relativistic light bending. To overcome this simplification, we have constructed an angle-dependent reflection model with the XILLVER code and self-consistently connected it with the relativistic blurring code RELLINE. The new model, relxill, calculates the proper emission angle of the radiation at each point on the accretion disk and then takes the corresponding reflection spectrum into account. We show that the reflected spectra from illuminated disks follow a limb-brightening law highly dependent on the ionization of disk and yet different from the commonly assumed form I∝ln (1 + 1/μ). A detailed comparison with the angle-averaged model is carried out in order to determine the bias in the parameters obtained by fitting a typical relativistic reflection spectrum. These simulations reveal that although the spin and inclination are mildly affected, the Fe abundance can be overestimated by up to a factor of two when derived from angle-averaged models. The fit of the new model to the Suzaku observation of the Seyfert galaxy Ark 120 clearly shows a significant improvement in the constraint of the physical parameters, in particular by enhancing the accuracy in the inclination angle and the spin determinations.

  11. A VLT/X-Shooter study of accretion and photoevaporation in Transitional Disks

    NASA Astrophysics Data System (ADS)

    Manara, Carlo Felice; Testi, Leonardo; Natta, Antonella; Ricci, Luca; Benisty, Myriam; Rosotti, Giovanni; Ercolano, Barbara

    2013-07-01

    Transitional Disks (TDs) are considered to be a late evolutionary stage of optically thick massive disks whose inner regions are being evacuated, leaving behind large holes that can be detected both by modeling the infrared spectral energy distribution (SED) or, in some cases, by mm-interferometry. These holes could be produced by processes of photoevaporation, grain growth, or planet formation. Still, none of these processes alone has been shown to be sufficient to explain all observations. In this context, the combination of inner hole size, mass accretion rate and wind properties is a powerful observational diagnostic of disk evolution models, but the current measurements of mass accretion rates for TDs are mostly based on secondary indicators (such as the 10% Ha width), and very few data on the wind properties for these objects are available. Here we present a detailed study of the accretion and wind properties of TDs carried out with the VLT/X-Shooter spectrograph. Combining new and archival X-Shooter observations, we collected a sample of more than 20 TDs from different nearby star-forming regions. Our sample includes objects with both small (<5-15 AU) and large (>20-30 AU) known inner hole size from the literature (either from mm-observations or infrared SED fitting). We check their stellar parameters (Teff, Lstar, Av, Mstar) and derive their accretion properties (Lacc, Macc) in a self-consistent way, which makes use of the wide wavelength coverage of X-Shooter, and study their wind properties by mean of different forbidden emission lines analysis. Here we present some preliminary results.

  12. A DOUBLE OUTBURST FROM IGR J00291+5934: IMPLICATIONS FOR ACCRETION DISK INSTABILITY THEORY

    SciTech Connect

    Hartman, Jacob M.; Galloway, Duncan K.; Chakrabarty, Deepto

    2011-01-01

    The accretion-powered millisecond pulsar IGR J00291+5934 underwent two {approx}10 day long outbursts during 2008, separated by 30 days in quiescence. Such a short quiescent period between outbursts has never been seen before from a neutron star X-ray transient. X-ray pulsations at the 599 Hz spin frequency are detected throughout both outbursts. For the first time, we derive a pulse phase model that connects two outbursts, providing a long baseline for spin frequency measurement. Comparison with the frequency measured during the 2004 outburst of this source gives a spin-down during quiescence of -(4 {+-} 1) x 10{sup -15} Hz s{sup -1}, approximately an order of magnitude larger than the long-term spin-down observed in the 401 Hz accretion-powered pulsar SAX J1808.4-3658. If this spin-down is due to magnetic dipole radiation, it requires a 2 x 10{sup 8} G field strength, and its high spin-down luminosity may be detectable with the Fermi Large Area Telescope. Alternatively, this large spin-down could be produced by gravitational wave emission from a fractional mass quadrupole moment of Q/I = 1 x 10{sup -9}. The rapid succession of the outbursts also provides a unique test of models for accretion in low-mass X-ray binaries. Disk instability models generally predict that an outburst will leave the accretion disk too depleted to fuel a second outburst after such a brief quiescence. We suggest a modification in which the outburst is shut off by the onset of a propeller effect before the disk is depleted. This model can explain the short quiescence and the unusually slow rise of the light curve of the second 2008 outburst.

  13. Orbital circularization of a planet accreting disk gas: the formation of distant jupiters in circular orbits based on a core accretion model

    SciTech Connect

    Kikuchi, Akihiro; Higuchi, Arika; Ida, Shigeru E-mail: higuchia@geo.titech.ac.jp

    2014-12-10

    Recently, gas giant planets in nearly circular orbits with large semimajor axes (a ∼ 30-1000 AU) have been detected by direct imaging. We have investigated orbital evolution in a formation scenario for such planets, based on a core accretion model. (1) Icy cores accrete from planetesimals at ≲ 30 AU, (2) they are scattered outward by an emerging nearby gas giant to acquire highly eccentric orbits, and (3) their orbits are circularized through the accretion of disk gas in outer regions, where they spend most of their time. We analytically derived equations to describe the orbital circularization through gas accretion. Numerical integrations of these equations show that the eccentricity decreases by a factor of more than 5 while the planetary mass increases by a factor of 10. Because runaway gas accretion increases planetary mass by ∼10-300, the orbits are sufficiently circularized. On the other hand, a is reduced at most only by a factor of two, leaving the planets in the outer regions. If the relative velocity damping by shock is considered, the circularization slows down, but is still efficient enough. Therefore, this scenario potentially accounts for the formation of observed distant jupiters in nearly circular orbits. If the apocenter distances of the scattered cores are larger than the disk sizes, their a shrink to a quarter of the disk sizes; the a-distribution of distant giants could reflect the outer edges of the disks in a similar way that those of hot jupiters may reflect inner edges.

  14. TURBULENCE IN THE OUTER REGIONS OF PROTOPLANETARY DISKS. I. WEAK ACCRETION WITH NO VERTICAL MAGNETIC FLUX

    SciTech Connect

    Simon, Jacob B.; Armitage, Philip J.; Beckwith, Kris; Bai, Xue-Ning; Stone, James M.

    2013-02-10

    We use local numerical simulations to investigate the strength and nature of magnetohydrodynamic (MHD) turbulence in the outer regions of protoplanetary disks, where ambipolar diffusion is the dominant non-ideal MHD effect. The simulations include vertical stratification and assume zero net vertical magnetic flux. We employ a super time-stepping technique to ameliorate the Courant restriction on the diffusive time step. We find that in idealized stratified simulations, with a spatially constant ambipolar Elsasser number Am, turbulence driven by the magnetorotational instability (MRI) behaves in a similar manner as in prior unstratified calculations. Turbulence dies away for Am {<=} 1, and becomes progressively more vigorous as ambipolar diffusion is decreased. Near-ideal MHD behavior is recovered for Am {>=} 10{sup 3}. In the intermediate regime (10 {<=} Am {<=} 10{sup 3}) ambipolar diffusion leads to substantial increases in both the period of the MRI dynamo cycle and the characteristic scales of magnetic field structures. To quantify the impact of ambipolar physics on disk accretion, we run simulations at 30 AU and 100 AU that include a vertical Am profile based upon far-ultraviolet (FUV) ionized disk models. These models develop a vertically layered structure analogous to the Ohmic dead zone that is present at smaller radii. We find that, although the levels of surface turbulence can be strong (and consistent with constraints on turbulent line widths at these radii), the inferred accretion rates are at least an order of magnitude smaller than those observed in T Tauri stars. This discrepancy is very likely due to the assumption of zero vertical magnetic field in our simulations and suggests that vertical magnetic fields are essential for MRI-driven accretion in the outer regions of protoplanetary disks.

  15. A Statistical Study of Accretion Disk Model Spectra for Cataclysmic Variables

    NASA Astrophysics Data System (ADS)

    Puebla, Raúl E.; Diaz, Marcos P.; Hubeny, Ivan

    2007-11-01

    We have performed a statistical test of the currently used accretion disk models for cataclysmic variables (CVs) using a set of 33 CVs with steady disks (10 old novae and 23 nova-like systems). The mass transfer rate () for each system was also calculated. Ultraviolet (UV) data were fitted by model spectra using a multiparametric optimization method, aiming to constrain the values. It was verified that these accretion disk models fail to fit both color and flux simultaneously, as previously noted when composite stellar atmosphere models were fitted to the UV spectra of CVs by Wade. By applying such models to a sample of novae and nova-like CVs, we confirm that the limb-darkening effect must be taken into account when estimating mass transfer rates, especially for high-inclination systems. Important fitting degeneracies of the basic disk parameters are analyzed. Our simulations suggest that to reproduce the observations a revision of the temperature profile, at least in the innermost parts of the disk, seems to be required, and possibly the vertical distribution of the viscosity should be revised. In addition, an optically thin layer or an extended disk component should be considered. This component may be physically represented by a disk wind and/or a chromosphere. A physical description of the emission-line profiles may help to break the degeneracies that appear when only the continuum is analyzed. The average value of found for nova-like systems is ~9.3 × 10-9 Modot yr-1, while ~1.3 × 10-8 Modot yr-1 is found for old classical novae. No clear evidence is found for either the presence or absence of a correlation between and the orbital period. Such correlation analysis was performed for high accretion rate systems (15 nova-like systems and 10 old novae), but we were not able to find a well-defined correlation as found by Patterson. By measuring the equivalent width of the emission lines (C IV λ1550 and He II λ1640) we found a lack of systems with low and

  16. Accretion Disk Coronal Heating Along The Z-pattern In Cygnus X-2

    NASA Astrophysics Data System (ADS)

    Schulz, Norbert S.; Huenemoerder, D. P.; Nowak, M.; Ji, L.; Yao, Y.; Canizares, C. R.

    2009-01-01

    Z-sources are low mass X-ray binaries containing accreting neutron stars which emit luminous X-rays near Eddington luminosities (L_Edd). Their X-ray spectra change in a systematic Z-shaped pattern from the horizontal (HB), to the normal (NB), and to the flaring branch (FB). Cyg X-2 is a prototype Z-source which we observed in X-rays twice along the entire X-ray spectral variation pattern at very high spectral resolution. We find a wealth of resolved and broad emission line features which we attribute to a hot accretion disk corona (ADC). The lines have Doppler velocities between 400 and 2700 km/s, which place the origin of emission within 10^10 cm of the accretion disk. We also find evidence for a flux evolution along the Z-pattern. The line emissivities indicate that the source luminosity changes from about 0.7 L_Edd in the HB to 2.6 L_Edd on the FB. The observed line flux evolution provides direct evidence for ongoing heating of the ADC along the spectral branches.

  17. Evidence for a Black Hole and Accretion Disk in the LINER NGC 4203.

    PubMed

    Shields; Rix; McIntosh; Ho; Rudnick; Filippenko; Sargent; Sarzi

    2000-05-01

    We present spectroscopic observations from the Hubble Space Telescope that reveal for the first time the presence of a broad pedestal of Balmer line emission in the LINER galaxy NGC 4203. The emission-line profile is suggestive of a relativistic accretion disk and is reminiscent of double-peaked transient Balmer emission observed in a handful of other LINERs. The very broad line emission thus constitutes clear qualitative evidence for a black hole, and spatially resolved narrow-line emission in NGC 4203 can be used to constrain its mass, MBHaccretion and hence with scenarios in which an ion torus irradiates an outer accretion disk that produces the observed double-peaked line emission. Follow-up observations will make it possible to improve the black hole mass estimate and study variability in the nuclear emission. PMID:10790063

  18. The Accretion Disk of the Lithium-Depleted Young Binary St 34

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee; Calvet, Nuria; Watson, Dan M.; D'Alessio, P.; Furlan, E.; Sargent, B.; Forrest, W. J.; Uchida, K. I.; Green, J. D.; Sloan, G. C.; Chen, C. H.; Najita, J.; Kemper, F.; Herter, T. L.; Morris, P.; Barry, D. J.; Hall, P.

    2005-01-01

    We presented the infrared spectrum of the young binary system St 34 obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The IRS spectrum clearly shows excess dust emission, consistent with the suggestion of White & Hillenbrand that St 34 is accreting from a circumbinary disk. The disk emission of St 34 is low in comparison with the levels observed in typical T Tauri stars; silicate features at 10 and 20 microns are much weaker than typically seen in T Tauri stars; and excess emission is nearly absent at the shortest wavelengths observed. These features of the infrared spectrum suggest substantial grain growth (to eliminate silicate features) and possible settling of dust to the disk midplane (to reduce the continuum excess emission levels), along with a relatively evacuated inner disk, as expected due to gravitational perturbations by the binary system. Although the position of St 34 in the H-R diagram suggests an age of 8f Myr, assuming that it lies at the distance of the Taurus-Auriga molecular clouds, White & Hillenbrand could not detect any Li I absorption, which would indicate a Li depletion age of roughly 25 Myr or more. We suggest that St 34 is closer than the Taurus clouds by about 30-40 pc and has an age roughly consistent with Li depletion models. Such an advanced age would make St 34 the oldest known low-mass pre-main-sequence object with a dusty accretion disk. The persistence of optically thick dust emission well outside the binary orbit may indicate a failure to make giant planets that could effectively remove dust particles.

  19. Advection/Diffusion of Large Scale Magnetic Field in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Lovelace, Richard V. E.; Rothstein, David M.; Bisnovatyi-Kogan, Gennady S.

    Winds and jets of proto-stellar systems are thought to arise from disk accretion involving (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) and (2) a large-scale magnetic field which gives rise to the winds and/or jets. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the surface layers of the disk are non-turbulent and thus highly conducting (or non-diffusive). This is because the MRI is suppressed in the surface layers where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity v r (z) which depends mainly on the midplane thermal to magnetic pressure ratio β > 1 and the magnetic Prandtl number of the turbulence P = viscosity/diffusivity. Boundary conditions at the disk surfaces take into account possible magnetic winds or jets and allow for a surface current flow in the highly conducting surface layers. The stationary solutions we find indicate that a weak (β > 1) large-scale field does not diffuse away as suggested by earlier work.

  20. VADER: A flexible, robust, open-source code for simulating viscous thin accretion disks

    NASA Astrophysics Data System (ADS)

    Krumholz, M. R.; Forbes, J. C.

    2015-06-01

    The evolution of thin axisymmetric viscous accretion disks is a classic problem in astrophysics. While models based on this simplified geometry provide only approximations to the true processes of instability-driven mass and angular momentum transport, their simplicity makes them invaluable tools for both semi-analytic modeling and simulations of long-term evolution where two- or three-dimensional calculations are too computationally costly. Despite the utility of these models, the only publicly-available frameworks for simulating them are rather specialized and non-general. Here we describe a highly flexible, general numerical method for simulating viscous thin disks with arbitrary rotation curves, viscosities, boundary conditions, grid spacings, equations of state, and rates of gain or loss of mass (e.g., through winds) and energy (e.g., through radiation). Our method is based on a conservative, finite-volume, second-order accurate discretization of the equations, which we solve using an unconditionally-stable implicit scheme. We implement Anderson acceleration to speed convergence of the scheme, and show that this leads to factor of ∼5 speed gains over non-accelerated methods in realistic problems, though the amount of speedup is highly problem-dependent. We have implemented our method in the new code Viscous Accretion Disk Evolution Resource (VADER), which is freely available for download from

  1. EFFECT OF BACKGROUND MAGNETIC FIELD ON TURBULENCE DRIVEN BY MAGNETOROTATIONAL INSTABILITY IN ACCRETION DISKS

    SciTech Connect

    Sai, Kazuhito; Katoh, Yuto; Terada, Naoki; Ono, Takayuki E-mail: yuto@stpp.gp.tohoku.ac.jp E-mail: ono@stpp.gp.tohoku.ac.jp

    2013-04-20

    We investigate the background magnetic field dependence of the saturated state of a magnetorotational instability (MRI) in an accretion disk by performing three-dimensional magnetohydrodynamic simulations. We assume an unstratified disk by employing the local shearing box approximation. Three different uniform background magnetic field configurations are treated for a wide range of field intensities. These simulations indicate that the time variations of the turbulent stress and the magnetic energy are altered by the presence of a poloidal component of the background field. We find that the saturation amplitude of the turbulent stress and the magnetic energy are determined by both the poloidal and azimuthal components of the field. In particular, when the poloidal component has the same intensity, the obtained turbulent stress for {beta}{sub y0} Almost-Equal-To 200 becomes smaller than those for a purely poloidal field case. Despite the fact that the background field affects the MRI turbulence, the correlation between the obtained turbulent stress and the magnetic energy in the nonlinear stage is independent of the field topology. Our results indicate that the saturated turbulent stress has a stronger correlation with the power of the perturbed component of the magnetic field than with the power of the total magnetic field. These results suggest that both the intensity and the direction of the background magnetic field significantly affect the turbulent motion of the MRI in accretion disks.

  2. Effect of Background Magnetic Field on Turbulence Driven by Magnetorotational Instability in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Sai, Kazuhito; Katoh, Yuto; Terada, Naoki; Ono, Takayuki

    2013-04-01

    We investigate the background magnetic field dependence of the saturated state of a magnetorotational instability (MRI) in an accretion disk by performing three-dimensional magnetohydrodynamic simulations. We assume an unstratified disk by employing the local shearing box approximation. Three different uniform background magnetic field configurations are treated for a wide range of field intensities. These simulations indicate that the time variations of the turbulent stress and the magnetic energy are altered by the presence of a poloidal component of the background field. We find that the saturation amplitude of the turbulent stress and the magnetic energy are determined by both the poloidal and azimuthal components of the field. In particular, when the poloidal component has the same intensity, the obtained turbulent stress for β y0 ≈ 200 becomes smaller than those for a purely poloidal field case. Despite the fact that the background field affects the MRI turbulence, the correlation between the obtained turbulent stress and the magnetic energy in the nonlinear stage is independent of the field topology. Our results indicate that the saturated turbulent stress has a stronger correlation with the power of the perturbed component of the magnetic field than with the power of the total magnetic field. These results suggest that both the intensity and the direction of the background magnetic field significantly affect the turbulent motion of the MRI in accretion disks.

  3. The spectra of relativistic accretion disks - Application to A0620-00

    NASA Technical Reports Server (NTRS)

    Fu, Albert; Taam, Ronald E.

    1990-01-01

    The X-ray flux emitted from a geometrically thin, relativistic accretion disk in the steady state approximation is investigated in order to place limits on the quiescent state mass flow rate in the soft X-ray transient black hole candidate source A0620-00. Specific attention is focused on the effects associated with gravitational redshifts, Doppler shifts, and on the enhancement of the apparent accretion disk area due to gravitational light bending on the continuum spectrum. It is found that the upper limit to the mass flow rate within the inner regions of the disk, constrained by the lack of soft X-rays in the quiescent state, is about 2.8 x 10 to the -11th solar mass/yr for black hole masses greater than about 5.4 solar mass. The optical data are consistent with these upper limits provided that the inclination angle of the binary system is less than about 65 deg. The upper limits and the lack of a hard X-ray flux, together, suggest that the soft X-ray transient model based upon a mass transfer instability situated in the stellar envelope of the companion is inapplicable to A0620-00.

  4. ACCRETION IN EVOLVED AND TRANSITIONAL DISKS IN CEP OB2: LOOKING FOR THE ORIGIN OF THE INNER HOLES

    SciTech Connect

    Sicilia-Aguilar, Aurora; Henning, Thomas; Hartmann, Lee W.

    2010-02-10

    We present accretion rates for a large number of solar-type stars in the Cep OB2 region, based on U-band observations. Our study comprises 95 members of the {approx}4 Myr old cluster Tr 37 (including 20 'transition' objects (TOs)), as well as the only classical T Tauri star (CTTS) in the {approx}12 Myr old cluster NGC 7160. The stars show different disk morphologies, with the majority of them having evolved and flattened disks. The typical accretion rates are about 1 order of magnitude lower than in regions aged 1-2 Myr, and we find no strong correlation between disk morphology and accretion rates. Although half of the TOs are not accreting, the median accretion rates of normal CTTS and accreting 'transition' disks are similar ({approx}3 x 10{sup -9} and 2 x 10{sup -9} M{sub sun} yr{sup -1}, respectively). Comparison with other regions suggests that the TOs observed at different ages do not necessarily represent the same type of objects, which is consistent with the fact that the different processes that can lead to reduced IR excess/inner disk clearing (e.g., binarity, dust coagulation/settling, photoevaporation, giant planet formation) do not operate on the same timescales. Accreting TOs in Tr 37 are probably suffering strong dust coagulation/settling. Regarding the equally large number of non-accreting TOs in the region, other processes, such as photoevaporation, the presence of stellar/substellar companions, and/or giant planet formation, may account for their 'transitional' spectral energy distributions and negligible accretion rates.

  5. The pulse amplitude variation with QPO frequency in SAX J1808.4-3658: Resonances with the accretion disk

    NASA Astrophysics Data System (ADS)

    Caliskan, Sirin; Alpar, Mehmet Ali; Sasmaz Mus, Sinem

    2016-07-01

    SAX J1808.4-3658 is an accreting millisecond pulsar with a spin period of 401 Hz. The pulsed amplitudes of this source vary with its kHz QPO frequencies (Bult & van der Klis 2015). The pulsed amplitude peaks at certain upper kHz QPO frequencies which we associate with boundary layer modes of the viscous accretion disk (Erkut et al. 2008). We model this as peaks in the energy dissipation rate at the accretion caps due to resonances between the accretion column and the driving modes of the boundary layer.

  6. C/O and Snowline Locations in Protoplanetary Disks: The Effect of Radial Drift and Viscous Gas Accretion

    NASA Astrophysics Data System (ADS)

    Piso, Ana-Maria

    2015-12-01

    The C/O ratio is a defining feature of both gas giant atmospheric and protoplanetary disk chemistry. In disks, the C/O ratio is regulated by the presence of snowlines of major volatiles at different distances from the central star. We explore the effect of radial drift of solids and viscous gas accretion onto the central star on the snowline locations of the main C and O carriers in a protoplanetary disk, H2O, CO2 and CO, and their consequences for the C/O ratio in gas and dust throughout the disk. We determine the snowline locations for a range of fixed initial particle sizes, in both an active and a passive disk. We find that grains with sizes ~0.5 cm < s < ~7 m for a passive disk and ~0.001 cm < s < ~7 m for an active disk desorb at a size-dependent location in the disk, which is independent of the particle's initial position. The snowline radius decreases for larger particles, up to sizes of ~7 m. Compared to a static disk, we find that radial drift and gas accretion move the H2O and CO2 snowlines inwards by up to 60 %, and the CO snowline by up to 50 %. We thus determine an inner limit on the snowline locations when radial drift and gas accretion are accounted for.

  7. LOCAL STUDY OF ACCRETION DISKS WITH A STRONG VERTICAL MAGNETIC FIELD: MAGNETOROTATIONAL INSTABILITY AND DISK OUTFLOW

    SciTech Connect

    Bai, Xue-Ning; Stone, James M.

    2013-04-10

    We perform three-dimensional, vertically-stratified, local shearing-box ideal MHD simulations of the magnetorotational instability (MRI) that include a net vertical magnetic flux, which is characterized by midplane plasma {beta}{sub 0} (ratio of gas to magnetic pressure). We have considered {beta}{sub 0} = 10{sup 2}, 10{sup 3}, and 10{sup 4}, and in the first two cases the most unstable linear MRI modes are well resolved in the simulations. We find that the behavior of the MRI turbulence strongly depends on {beta}{sub 0}: the radial transport of angular momentum increases with net vertical flux, achieving {alpha} {approx} 0.08 for {beta} = 10{sup 4} and {alpha} {approx}> 1.0 for {beta}{sub 0} = 100, where {alpha} is the height-integrated and mass-weighted Shakura-Sunyaev parameter. A critical value lies at {beta}{sub 0} {approx} 10{sup 3}: for {beta}{sub 0} {approx}> 10{sup 3}, the disk consists of a gas pressure dominated midplane and a magnetically dominated corona. The turbulent strength increases with net flux, and angular momentum transport is dominated by turbulent fluctuations. The magnetic dynamo that leads to cyclic flips of large-scale fields still exists, but becomes more sporadic as net flux increases. For {beta}{sub 0} {approx}< 10{sup 3}, the entire disk becomes magnetically dominated. The turbulent strength saturates, and the magnetic dynamo is fully quenched. Stronger large-scale fields are generated with increasing net flux, which dominates angular momentum transport. A strong outflow is launched from the disk by the magnetocentrifugal mechanism, and the mass flux increases linearly with net vertical flux and shows sign of saturation at {beta}{sub 0} {approx}< 10{sup 2}. However, the outflow is unlikely to be directly connected to a global wind: for {beta}{sub 0} {approx}> 10{sup 3}, the large-scale field has no permanent bending direction due to dynamo activities, while for {beta}{sub 0} {approx}< 10{sup 3}, the outflows from the top and bottom

  8. The Structure of the Accretion Disk in the Lensed Quasar SBS 0909+532

    NASA Astrophysics Data System (ADS)

    Mediavilla, E.; Muñoz, J. A.; Kochanek, C. S.; Guerras, E.; Acosta-Pulido, J.; Falco, E.; Motta, V.; Arribas, S.; Manchado, A.; Mosquera, A.

    2011-03-01

    We derive the size and temperature profile of the accretion disk of the lensed quasar SBS 0909+532 by measuring the wavelength dependence (chromaticity) of the microlensing magnification produced by the stars in the lens galaxy. After correcting for extinction using the flux ratios of 14 emission lines, we observe a marked change in the B-A flux ratio with wavelength, varying from -0.67 ± 0.05 mag at (rest frame) ~1460 Å to -0.24 ± 0.07 mag at ~6560 Å. For λ >~ 7000 Å both effects, extinction and microlensing, look minimal. Simulations indicate that image B rather than A is strongly microlensed. If we model the change in disk size from 1460 Å to 6560 Å using a Gaussian source (I vprop exp(-R 2/2r 2 s )) with a disk size scaling with wavelength as rs vprop λ p , we find rs = 7+5 -3 light-days at 1460 Å and p = 0.9+0.6 -0.3 for uniform priors on rs and p, and rs = 4+3 -3 light-days and p = 1.0+0.6 -0.4 for a logarithmic prior on rs . The disk temperature profile T vprop R -1/p is consistent with thin disk theory (T vprop R -3/4), given the uncertainties. The estimates of rs are also in agreement with the size inferred from thin disk theory using the estimated black hole mass (M BH ~= 2 × 109 M sun) but not with the smaller size estimated from thin disk theory and the optical flux. We also use the flux ratios of the unmicrolensed emission lines to determine the extinction curve of the dust in the lens galaxy, finding that it is similar to that of the LMC2 Supershell.

  9. Nonlinear evolution of tidally distorted accretion disks: Two-dimensional simulations

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Goodman, Jeremy

    1994-01-01

    According to a previously published linear analysis, the tidal distortion of accretion disks in binary star systems produces a local hydrodynamic instability to m = 1 internal waves, which may have arbitrarily small wavelengths in the absence of viscosity. The instability is three-dimensional and approximately incrompressible. To explore the nonlinear outcome of this instability, we develop a shearing-sheet approximation on scales comparable to the disk thickness. The large-scale azimuthal variation of the disk is represented by varying the local metric with time (local orbital phase). The hydrodynamic equations can then be posed two-dimensionally on local meridional planes. We solve these equations with a second-order gasdynamical code based on the Total-Variation-Diminishing scheme. Our simulations confirm the predicted linear growth rate. The modes saturate chaotically at velocities scaling as the product of the linear growth rate and the wavelength. If the wavelength is small compared with the disk thickness, the modes remain nearly incompressible even when nonlinear. The two-dimensional power spectrum of velocities after saturation is roughly isotropic and extends over a broad range of scales in an approximately power-law fashion. We measure the heating rate associated with the nonlinear dissipation of the modes. The dissipation implies a secular torque on the disk and a return of angular momentum to the secondary star via the tidal potential. The estimated torque is somewhat larger than the tidal torque produced by maximal disk viscosity (alpha approximately 1). At least in these two-dimensional simulations, however, there is no significant angular momentum flux within the disk.

  10. PATCHY ACCRETION DISKS IN ULTRA-LUMINOUS X-RAY SOURCES

    SciTech Connect

    Miller, J. M.; Bachetti, M.; Barret, D.; Webb, N. A.; Harrison, F. A.; Walton, D. J.; Rana, V.; Fabian, A. C.

    2014-04-10

    The X-ray spectra of the most extreme ultra-luminous X-ray sources—those with L ≥ 10{sup 40} erg s{sup –1}—remain something of a mystery. Spectral roll-over in the 5-10 keV band was originally detected in the deepest XMM-Newton observations of the brightest sources; this is confirmed in subsequent NuSTAR spectra. This emission can be modeled via Comptonization, but with low electron temperatures (kT{sub e} ≅ 2 keV) and high optical depths (τ ≅ 10) that pose numerous difficulties. Moreover, evidence of cooler thermal emission that can be fit with thin disk models persists, even in fits to joint XMM-Newton and NuSTAR observations. Using NGC 1313 X-1 as a test case, we show that a patchy disk with a multiple temperature profile may provide an excellent description of such spectra. In principle, a number of patches within a cool disk might emit over a range of temperatures, but the data only require a two-temperature profile plus standard Comptonization, or three distinct blackbody components. A mechanism such as the photon bubble instability may naturally give rise to a patchy disk profile, and could give rise to super-Eddington luminosities. It is possible, then, that a patchy disk (rather than a disk with a standard single-temperature profile) might be a hallmark of accretion disks close to or above the Eddington limit. We discuss further tests of this picture and potential implications for sources such as narrow-line Seyfert-1 galaxies and other low-mass active galactic nuclei.

  11. Fine-Tuning the Accretion Disk Clock in Hercules X-1

    NASA Technical Reports Server (NTRS)

    Still, M.; Boyd, P.

    2004-01-01

    RXTE ASM count rates from the X-ray pulsar Her X-1 began falling consistently during the late months of 2003. The source is undergoing another state transition similar to the anomalous low state of 1999. This new event has triggered observations from both space and ground-based observatories. In order to aid data interpretation and telescope scheduling, and to facilitate the phase-connection of cycles before and after the state transition, we have re-calculated the precession ephemeris using cycles over the last 3.5 years. We report that the source has displayed a different precession period since the last anomalous event. Additional archival data from CGRO suggests that each low state is accompanied by a change in precession period and that the subsequent period is correlated with accretion flux. Consequently our analysis reveals long-term accretion disk behaviour which is predicted by theoretical models of radiation-driven warping.

  12. A VLT/X-Shooter study of accretion and photoevaporation in Transitional Disks

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Testi, L.; Natta, A.; Ricci, L.; Benisty, M.; Rosotti, G.; Ercolano, B.

    2014-01-01

    We present preliminary results of a detailed study of the accretion, stellar, and wind properties of transitional disks (TDs) carried out with the X-Shooter spectrograph. Combining new and archival spectra, we collected a sample of more than 20 TDs from different nearby star-forming regions. Our sample includes objects with both small (<5-15 AU) and large (>20-30 AU) known inner hole size from the literature (either from mm-observations or IR SED fitting). We check their stellar parameters (Teff, L*, A V , M *) and derive their accretion properties (Lacc, Ṁ acc) in a self-consistent way, which makes use of the wide wavelength coverage of X-Shooter, and study their wind properties by mean of different forbidden emission lines analysis.

  13. A global three-dimensional radiation magneto-hydrodynamic simulation of super-eddington accretion disks

    SciTech Connect

    Jiang, Yan-Fei; Stone, James M.; Davis, Shane W.

    2014-12-01

    We study super-Eddington accretion flows onto black holes using a global three-dimensional radiation magneto-hydrodynamical simulation. We solve the time-dependent radiative transfer equation for the specific intensities to accurately calculate the angular distribution of the emitted radiation. Turbulence generated by the magneto-rotational instability provides self-consistent angular momentum transfer. The simulation reaches inflow equilibrium with an accretion rate ∼220 L {sub Edd}/c {sup 2} and forms a radiation-driven outflow along the rotation axis. The mechanical energy flux carried by the outflow is ∼20% of the radiative energy flux. The total mass flux lost in the outflow is about 29% of the net accretion rate. The radiative luminosity of this flow is ∼10 L {sub Edd}. This yields a radiative efficiency ∼4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in most models, such as the slim disk model, which neglect vertical advection. Our inferred radiative efficiencies also exceed published results from previous global numerical simulations, which did not attribute a significant role to vertical advection. We briefly discuss the implications for the growth of supermassive black holes in the early universe and describe how these results provided a basis for explaining the spectrum and population statistics of ultraluminous X-ray sources.

  14. A Global Three-dimensional Radiation Magneto-hydrodynamic Simulation of Super-Eddington Accretion Disks

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei; Stone, James M.; Davis, Shane W.

    2014-12-01

    We study super-Eddington accretion flows onto black holes using a global three-dimensional radiation magneto-hydrodynamical simulation. We solve the time-dependent radiative transfer equation for the specific intensities to accurately calculate the angular distribution of the emitted radiation. Turbulence generated by the magneto-rotational instability provides self-consistent angular momentum transfer. The simulation reaches inflow equilibrium with an accretion rate ~220 L Edd/c 2 and forms a radiation-driven outflow along the rotation axis. The mechanical energy flux carried by the outflow is ~20% of the radiative energy flux. The total mass flux lost in the outflow is about 29% of the net accretion rate. The radiative luminosity of this flow is ~10 L Edd. This yields a radiative efficiency ~4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in most models, such as the slim disk model, which neglect vertical advection. Our inferred radiative efficiencies also exceed published results from previous global numerical simulations, which did not attribute a significant role to vertical advection. We briefly discuss the implications for the growth of supermassive black holes in the early universe and describe how these results provided a basis for explaining the spectrum and population statistics of ultraluminous X-ray sources.

  15. Linear dynamics of weakly viscous accretion disks: a disk analog of Tollmien-Schlichting waves

    NASA Astrophysics Data System (ADS)

    Umurhan, O. M.; Shaviv, G.

    2009-04-01

    This paper discusses new perspectives and approaches to the problem of disk dynamics where, in this study, we focus on the effects of the viscous instabilities influenced by boundary effects. The Boussinesq approximation of the viscous large shearing box equations is analyzed in which the azimuthal length scale of the disturbance is much larger than the radial and vertical scales. We examine the stability of a non-axisymmetric potential vorticity mode, i.e. a PV-anomaly. in a configuration in which buoyant convection and the strato-rotational instability do not to operate. We consider a series of boundary conditions that show the PV-anomaly to be unstable both on finite and semi-infinite radial domains. We find these conditions lead to an instability that is the disk analog of Tollmien-Schlichting waves. When the viscosity is weak, evidence of the instability is most pronounced by the emergence of a vortex sheet at the critical layer located away from the boundary where the instability is generated. For some boundary conditions, a necessary criterion for the onset of instability for vertical wavelengths that are a sizable fraction of the layer's thickness and when the viscosity is low is that the appropriate Froude number of the flow be greater than one. This instability persists if more realistic boundary conditions are applied, although the criterion on the Froude number is more complicated. The unstable waves studied here share qualitative features to the instability seen in rotating Blasius boundary layers. The implications of these results are discussed. An overall new strategy for exploring and interpreting disk instability mechanisms is also suggested.

  16. OBSERVATIONAL LIMITS ON TYPE 1 ACTIVE GALACTIC NUCLEUS ACCRETION RATE IN COSMOS

    SciTech Connect

    Trump, Jonathan R.; Impey, Chris D.; Gabor, Jared; Kelly, Brandon C.; Elvis, Martin; Hao Heng; Huchra, John P.; Merloni, Andrea; Bongiorno, Angela; Brusa, Marcella; Cappelluti, Nico; McCarthy, Patrick J.; Koekemoer, Anton; Nagao, Tohru; Salvato, Mara; Scoville, Nick Z.

    2009-07-20

    We present black hole masses and accretion rates for 182 Type 1 active galactic nuclei (AGNs) in COSMOS. We estimate masses using the scaling relations for the broad H {beta}, Mg II, and C IV emission lines in the redshift ranges 0.16 < z < 0.88, 1 < z < 2.4, and 2.7 < z < 4.9. We estimate the accretion rate using an Eddington ratio L{sub I}/L{sub Edd} estimated from optical and X-ray data. We find that very few Type 1 AGNs accrete below L{sub I} /L{sub Edd} {approx} 0.01, despite simulations of synthetic spectra which show that the survey is sensitive to such Type 1 AGNs. At lower accretion rates the broad-line region may become obscured, diluted, or nonexistent. We find evidence that Type 1 AGNs at higher accretion rates have higher optical luminosities, as more of their emission comes from the cool (optical) accretion disk with respect to shorter wavelengths. We measure a larger range in accretion rate than previous works, suggesting that COSMOS is more efficient at finding low accretion rate Type 1 AGNs. However, the measured range in accretion rate is still comparable to the intrinsic scatter from the scaling relations, suggesting that Type 1 AGNs accrete at a narrow range of Eddington ratio, with L{sub I} /L{sub Edd} {approx} 0.1.

  17. Accretion disks around neutron and strange stars in R+aR2 gravity

    NASA Astrophysics Data System (ADS)

    Staykov, Kalin V.; Doneva, Daniela D.; Yazadjiev, Stoytcho S.

    2016-08-01

    We study the electromagnetic spectrum of accretion disks around neutron and strange stars in R+aR2 gravity. Both static and rapidly rotating models are investigated. The results are compared with the General Relativistic results. We found difference between the results in both theories of about 50% for the electromagnetic flux and about 20% in the luminosity for models with equal mass and angular velocity in both theories. The observed differences are much lower for models rotating with Keplerian velocity and with equal masses.

  18. Beltrami state in black-hole accretion disk: A magnetofluid approach

    NASA Astrophysics Data System (ADS)

    Bhattacharjee, Chinmoy; Das, Rupam; Stark, David J.; Mahajan, S. M.

    2015-12-01

    Using the magnetofluid unification framework, we show that the accretion disk plasma (embedded in the background geometry of a black hole) can relax to a class of states known as the Beltrami-Bernoulli (BB) equilibria. Modeling the disk plasma as a Hall magnetohydrodynamics (MHD) system, we find that the space-time curvature can significantly alter the magnetic (velocity) decay rates as we move away from the compact object; the velocity profiles in BB states, for example, deviate substantially from the predicted corresponding geodesic velocity profiles. These departures imply a rich interplay of plasma dynamics and general relativity revealed by examining the corresponding Bernoulli condition representing "homogeneity" of total energy. The relaxed states have their origin in the constraints provided by the two helicity invariants of Hall MHD. These helicities conspire to introduce an oscillatory length scale into the system that is strongly influenced by relativistic and thermal effects.

  19. A ROBUST DETERMINATION OF THE SIZE OF QUASAR ACCRETION DISKS USING GRAVITATIONAL MICROLENSING

    SciTech Connect

    Jimenez-Vicente, J.; Mediavilla, E.; Kochanek, C. S.

    2012-06-01

    Using microlensing measurements for a sample of 27 image pairs of 19 lensed quasars we determine a maximum likelihood estimate for the accretion disk size of an average quasar of r{sub s} = 4.0{sup +2.4}{sub -3.1} lt-day at rest frame ({lambda}) = 1736 Angstrom-Sign for microlenses with a mean mass of (M) = 0.3 M{sub Sun }. This value, in good agreement with previous results from smaller samples, is roughly a factor of five greater than the predictions of the standard thin disk model. The individual size estimates for the 19 quasars in our sample are also in excellent agreement with the results of the joint maximum likelihood analysis.

  20. Beltrami state in black-hole accretion disk: A magnetofluid approach.

    PubMed

    Bhattacharjee, Chinmoy; Das, Rupam; Stark, David J; Mahajan, S M

    2015-12-01

    Using the magnetofluid unification framework, we show that the accretion disk plasma (embedded in the background geometry of a black hole) can relax to a class of states known as the Beltrami-Bernoulli (BB) equilibria. Modeling the disk plasma as a Hall magnetohydrodynamics (MHD) system, we find that the space-time curvature can significantly alter the magnetic (velocity) decay rates as we move away from the compact object; the velocity profiles in BB states, for example, deviate substantially from the predicted corresponding geodesic velocity profiles. These departures imply a rich interplay of plasma dynamics and general relativity revealed by examining the corresponding Bernoulli condition representing "homogeneity" of total energy. The relaxed states have their origin in the constraints provided by the two helicity invariants of Hall MHD. These helicities conspire to introduce an oscillatory length scale into the system that is strongly influenced by relativistic and thermal effects. PMID:26764835

  1. Reprocessing of Soft X-ray Emission Lines in Black Hole Accretion Disks

    SciTech Connect

    Mauche, C W; Liedahl, D A; Mathiesen, B F; Jimenez-Garate, M A; Raymond, J C

    2003-10-17

    By means of a Monte Carlo code that accounts for Compton scattering and photoabsorption followed by recombination, we have investigated the radiation transfer of Ly{alpha}, He{alpha}, and recombination continua photons of H- and He-like C, N, O, and Ne produced in the photoionized atmosphere of a relativistic black hole accretion disk. We find that photoelectric opacity causes significant attenuation of photons with energies above the O VIII K-edge; that the conversion efficiencies of these photons into lower-energy lines and recombination continua are high; and that accounting for this reprocessing significantly (by factors of 21% to 105%) increases the flux of the Ly{alpha} and He{alpha} emission lines of H- and He-like C and O escaping the disk atmosphere.

  2. Beltrami state in black-hole accretion disk: A magnetofluid approach.

    PubMed

    Bhattacharjee, Chinmoy; Das, Rupam; Stark, David J; Mahajan, S M

    2015-12-01

    Using the magnetofluid unification framework, we show that the accretion disk plasma (embedded in the background geometry of a black hole) can relax to a class of states known as the Beltrami-Bernoulli (BB) equilibria. Modeling the disk plasma as a Hall magnetohydrodynamics (MHD) system, we find that the space-time curvature can significantly alter the magnetic (velocity) decay rates as we move away from the compact object; the velocity profiles in BB states, for example, deviate substantially from the predicted corresponding geodesic velocity profiles. These departures imply a rich interplay of plasma dynamics and general relativity revealed by examining the corresponding Bernoulli condition representing "homogeneity" of total energy. The relaxed states have their origin in the constraints provided by the two helicity invariants of Hall MHD. These helicities conspire to introduce an oscillatory length scale into the system that is strongly influenced by relativistic and thermal effects.

  3. Saturation of the Magneto-rotational Instability in Strongly Radiation-dominated Accretion Disks

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei; Stone, James M.; Davis, Shane W.

    2013-04-01

    The saturation level of the magneto-rotational instability (MRI) in a strongly radiation-dominated accretion disk is studied using a new Godunov radiation MHD code in the unstratified shearing box approximation. Since vertical gravity is neglected in this work, our focus is on how the MRI saturates in the optically thick mid-plane of the disk. We confirm that turbulence generated by the MRI is very compressible in the radiation-dominated regime, as found by previous calculations using the flux-limited diffusion approximation. We also find little difference in the saturation properties in calculations that use a larger horizontal domain (up to four times the vertical scale height in the radial direction). However, in strongly radiation pressure dominated disks (one in which the radiation energy density reaches ~1% of the rest mass energy density of the gas), we find that Maxwell stress from the MRI turbulence is larger than the value produced when radiation pressure is replaced with the same amount of gas pressure. At the same time, the ratio between Maxwell stress and Reynolds stress is increased by almost a factor of eight compared with the gas pressure dominated case. We suggest that this effect is caused by radiation drag, which acts like bulk viscosity and changes the effective magnetic Prandtl number of the fluid. Radiation viscosity significantly exceeds both the microscopic plasma viscosity and resistivity, ensuring that radiation-dominated systems occupy the high magnetic Prandtl number regime. Nevertheless, we find that radiative shear viscosity is negligible compared to the Maxwell stress and Reynolds stress in the flow. This may have important implications for the structure of radiation-dominated accretion disks.

  4. A Method for the Study of Accretion Disk Emission in Cataclysmic Variables. I. The Model

    NASA Astrophysics Data System (ADS)

    Puebla, Raúl E.; Diaz, Marcos P.; Hillier, D. John; Hubeny, Ivan

    2011-07-01

    We have developed a spectrum synthesis method for modeling the ultraviolet (UV) emission from the accretion disk from cataclysmic variables (CVs). The disk is separated into concentric rings, with an internal structure from the Wade & Hubeny disk-atmosphere models. For each ring, a wind atmosphere is calculated in the comoving frame with a vertical velocity structure obtained from a solution of the Euler equation. Using simple assumptions, regarding rotation and the wind streamlines, these one-dimensional models are combined into a single 2.5-dimensional model for which we compute synthetic spectra. We find that the resulting line and continuum behavior as a function of the orbital inclination is consistent with the observations, and verify that the accretion rate affects the wind temperature, leading to corresponding trends in the intensity of UV lines. In general, we also find that the primary mass has a strong effect on the P Cygni absorption profiles, the synthetic emission line profiles are strongly sensitive to the wind temperature structure, and an increase in the mass-loss rate enhances the resonance line intensities. Synthetic spectra were compared with UV data for two high orbital inclination nova-like CVs—RW Tri and V347 Pup. We needed to include disk regions with arbitrary enhanced mass loss to reproduce reasonably well widths and line profiles. This fact and a lack of flux in some high ionization lines may be the signature of the presence of density-enhanced regions in the wind, or alternatively, may result from inadequacies in some of our simplifying assumptions.

  5. DISSIPATION AND VERTICAL ENERGY TRANSPORT IN RADIATION-DOMINATED ACCRETION DISKS

    SciTech Connect

    Blaes, Omer; Shabaltas, Natalia; Krolik, Julian H.; Hirose, Shigenobu

    2011-06-01

    Standard models of radiation-supported accretion disks generally assume that diffusive radiation flux is solely responsible for vertical heat transport. This requires that heat must be generated at a critical rate per unit volume if the disk is to be in hydrostatic and thermal equilibrium. This raises the question of how heat is generated and how energy is transported in MHD turbulence. By analysis of a number of radiation/MHD stratified shearing-box simulations, we show that the divergence of the diffusive radiation flux is indeed capped at the critical rate, but deep inside the disk, substantial vertical energy flux is also carried by advection of radiation. Work done by radiation pressure is a significant part of the energy budget, and much of this work is dissipated later through damping by radiative diffusion. We show how this damping can be measured in the simulations and identify its physical origins. Radiative damping accounts for as much as tens of percent of the total dissipation and is the only realistic physical mechanism for dissipation of turbulence that can actually be resolved in numerical simulations of accretion disks. Buoyancy associated with dynamo-driven, highly magnetized, nearly isobaric nonlinear slow magnetosonic fluctuations is responsible for the radiation advection flux and also explains the persistent periodic magnetic upwelling seen at all values of the radiation to gas pressure ratio. The intimate connection between radiation advection and magnetic buoyancy is the first example we know of in astrophysics in which a dynamo has direct impact on the global energetics of a system.

  6. SATURATION OF THE MAGNETO-ROTATIONAL INSTABILITY IN STRONGLY RADIATION-DOMINATED ACCRETION DISKS

    SciTech Connect

    Jiang Yanfei; Stone, James M.; Davis, Shane W.

    2013-04-20

    The saturation level of the magneto-rotational instability (MRI) in a strongly radiation-dominated accretion disk is studied using a new Godunov radiation MHD code in the unstratified shearing box approximation. Since vertical gravity is neglected in this work, our focus is on how the MRI saturates in the optically thick mid-plane of the disk. We confirm that turbulence generated by the MRI is very compressible in the radiation-dominated regime, as found by previous calculations using the flux-limited diffusion approximation. We also find little difference in the saturation properties in calculations that use a larger horizontal domain (up to four times the vertical scale height in the radial direction). However, in strongly radiation pressure dominated disks (one in which the radiation energy density reaches {approx}1% of the rest mass energy density of the gas), we find that Maxwell stress from the MRI turbulence is larger than the value produced when radiation pressure is replaced with the same amount of gas pressure. At the same time, the ratio between Maxwell stress and Reynolds stress is increased by almost a factor of eight compared with the gas pressure dominated case. We suggest that this effect is caused by radiation drag, which acts like bulk viscosity and changes the effective magnetic Prandtl number of the fluid. Radiation viscosity significantly exceeds both the microscopic plasma viscosity and resistivity, ensuring that radiation-dominated systems occupy the high magnetic Prandtl number regime. Nevertheless, we find that radiative shear viscosity is negligible compared to the Maxwell stress and Reynolds stress in the flow. This may have important implications for the structure of radiation-dominated accretion disks.

  7. Do Circumnuclear Dense Gas Disks Drive Mass Accretion onto Supermassive Black Holes?

    NASA Astrophysics Data System (ADS)

    Izumi, Takuma; Kawakatu, Nozomu; Kohno, Kotaro

    2016-08-01

    We present a positive correlation between the mass of dense molecular gas ({M}{{dense}}) of ˜100 pc scale circumnuclear disks (CNDs) and the black hole mass accretion rate ({\\dot{M}}{{BH}}) in a total of 10 Seyfert galaxies, based on data compiled from the literature and an archive (median aperture θ med = 220 pc). A typical {M}{{dense}} of CNDs is 107–8 {M}ȯ , estimated from the luminosity of the dense gas tracer, the HCN(1–0) emission line. Because dense molecular gas is the site of star formation, this correlation is virtually equivalent to the one between the nuclear star-formation rate and {\\dot{M}}{{BH}} revealed previously. Moreover, the {M}{{dense}}{--}{\\dot{M}}{{BH}} correlation was tighter for CND-scale gas than for the gas on kiloparsec or larger scales. This indicates that CNDs likely play an important role in fueling black holes, whereas greater than kiloparesec scale gas does not. To demonstrate a possible approach for studying the CND-scale accretion process with the Atacama Large Millimeter/submillimeter Array, we used a mass accretion model where angular momentum loss due to supernova explosions is vital. Based on the model prediction, we suggest that only the partial fraction of the mass accreted from the CND ({\\dot{M}}{{acc}}) is consumed as {\\dot{M}}{{BH}}. However, {\\dot{M}}{{acc}} agrees well with the total nuclear mass flow rate (i.e., {\\dot{M}}{{BH}} + outflow rate). Although these results are still tentative with large uncertainties, they support the view that star formation in CNDs can drive mass accretion onto supermassive black holes in Seyfert galaxies.

  8. Do Circumnuclear Dense Gas Disks Drive Mass Accretion onto Supermassive Black Holes?

    NASA Astrophysics Data System (ADS)

    Izumi, Takuma; Kawakatu, Nozomu; Kohno, Kotaro

    2016-08-01

    We present a positive correlation between the mass of dense molecular gas ({M}{{dense}}) of ˜100 pc scale circumnuclear disks (CNDs) and the black hole mass accretion rate ({\\dot{M}}{{BH}}) in a total of 10 Seyfert galaxies, based on data compiled from the literature and an archive (median aperture θ med = 220 pc). A typical {M}{{dense}} of CNDs is 107-8 {M}⊙ , estimated from the luminosity of the dense gas tracer, the HCN(1-0) emission line. Because dense molecular gas is the site of star formation, this correlation is virtually equivalent to the one between the nuclear star-formation rate and {\\dot{M}}{{BH}} revealed previously. Moreover, the {M}{{dense}}{--}{\\dot{M}}{{BH}} correlation was tighter for CND-scale gas than for the gas on kiloparsec or larger scales. This indicates that CNDs likely play an important role in fueling black holes, whereas greater than kiloparesec scale gas does not. To demonstrate a possible approach for studying the CND-scale accretion process with the Atacama Large Millimeter/submillimeter Array, we used a mass accretion model where angular momentum loss due to supernova explosions is vital. Based on the model prediction, we suggest that only the partial fraction of the mass accreted from the CND ({\\dot{M}}{{acc}}) is consumed as {\\dot{M}}{{BH}}. However, {\\dot{M}}{{acc}} agrees well with the total nuclear mass flow rate (i.e., {\\dot{M}}{{BH}} + outflow rate). Although these results are still tentative with large uncertainties, they support the view that star formation in CNDs can drive mass accretion onto supermassive black holes in Seyfert galaxies.

  9. Local magnetohydrodynamic instabilities and the wave-driven dynamo in accretion disks

    NASA Technical Reports Server (NTRS)

    Vishniac, Ethan T.; Diamond, Patrick

    1992-01-01

    We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the strength and organization of the magnetic field in a thin accretion disk. We discuss a model in which the wave-driven dynamo growth rate is balanced by the dissipative effects of the MSI. As in earlier work, the net helicity is due to small advective motions driven by nonlinear interactions between internal waves. Assuming a simple model of the internal wave spectrum generated from the primary m = 1 internal waves, we find that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (where H is the disk thickness and r is its radius). On very small scales the shearing instability will produce an isotropic fluctuating field. For a stationary disk this is equivalent to a dimensionless 'viscosity' of about (H/r) exp 4/3. The vertical and radial diffusion coefficients will be comparable to each other. Magnetic buoyancy will be largely suppressed by the turbulence due to the MSI. We present a rough estimate of its effects and find that it removes magnetic flux from the disk at a rate comparable to that caused by turbulent diffusion.

  10. Polarization of Line Emission from an Accretion Disk and Application to ARP 102B

    NASA Technical Reports Server (NTRS)

    Chen, Kaiyou; Halpern, Jules P.; Titarchuk, Lev G.

    1997-01-01

    We model the polarization properties of line emission from an accretion disk under a range of assumptions about the source function and electron-scattering optical depth tau(sub es). For small values of tau(sub es) and modest viewing angles, polarization can be in excess of the Chandrasekhar result for tau(sub es) = infinity. The polarization vector can be either parallel or perpendicular to the projected direction of the disk axis. The polarization properties of the double-peaked H-alpha emission line of the broad-line radio galaxy Arp 102B observed by Antonucci, Hurt, and Agol can be understood in terms of electron scattering and line broadening within the line-emitting region if tau(sub es) is of order unity, and if the position angle of polarization is parallel to the projected disk axis. The required small tau(sub es) is consistent with the hypothesis that the Balmer lines in Arp 102B are produced by photoionization of the disk atmosphere.

  11. EVOLUTION OF WARPED ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI. I. ROLES OF FEEDING AT THE OUTER BOUNDARIES

    SciTech Connect

    Li, Yan-Rong; Wang, Jian-Min; Cheng, Cheng; Qiu, Jie

    2013-02-10

    We investigate the alignment processes of spinning black holes and their surrounding warped accretion disks in a frame of two different types of feeding at the outer boundaries. We consider (1) fixed flows in which gas is continually fed with a preferred angular momentum, and (2) free flows in which there is no gas supply and the disks diffuse freely at their outer edges. As expected, we find that for the cases of fixed flows the black hole disk systems always align on timescales of several 10{sup 6} yr, irrespective of the initial inclinations. If the initial inclination angles are larger than {pi}/2, the black hole accretion transits from retrograde to prograde fashion, and the accreted mass onto the black holes during these two phases is comparable. On the other hand, for the cases of free flows, both alignments and anti-alignments can occur, depending on the initial inclinations and the ratios of the angular momentum of the disks to that of the black holes. In such cases, the disks will be consumed within timescales of 10{sup 6} yr by black holes accreting at the Eddington limit. We propose that there is a close connection between the black hole spin and the lifetime for which the feeding persists, which determines the observable episodic lifetimes of active galactic nuclei. We conclude that careful inclusion of the disk feeding at the outer boundaries is crucial for modeling the evolution of the black hole spin.

  12. Excitation and Damping of Acoustic Waves in Three-Dimensional Accretion Disks

    NASA Astrophysics Data System (ADS)

    Mosqueira, I.; Houben, H.

    2002-09-01

    The damping of acoustic waves plays a key role in determining the criterion for gap opening (see Estrada and Mosqueira, this conference). Because of the potential significance of gap-opening in regulating the rate of accretion and of radial migration, it is important to investigate all sources of wave damping in an accretion disk. Here we mainly discuss damping mechanisms arising from 3-D effects in disks with aspect ratio H/r ~ 0.1. A 2-D treatment is valid when the response of the disk is locally isothermal with adiabatic index γ = 1 and the vertical forcing is ignored. In that case, tidal forcing will generate a 2-D wave which is likely to damp due to wave steepening in a lengthscale of order ~ rL (with weak dependence on the mass of the pertuber), where rL is the radial location of the Lindblad resonance where the acoustic wave is launched (Goodman and Rafikov 2001; Rafikov 2002). On the other hand, vertically thermally stratified disks, as may be the case for active disks with high-optical depth, generate 3-D waves which damp due to non-linear dissipation in a lengthscale of order ~ rL/m, where m is the azimuthal wavenumber (Lubow and Ogilvie 1998). In this case, most of the angular momentum flux is carried by the f-mode; however, tidal forcing also excites other modes. Finally, in a vertically isothermal (but not radially) disk with γ = 5/3 horizontal tidal forcing excites buoyancy g waves that receive ~ 20 per cent of the energy flux for a m = 0 mode (Bate et al. 2002). These g-waves have non-zero vertical group velocity, and are excited primarily away from the midplane, where non-linear dissipation is more readily attained. The radial damping length for these waves is likely to be ~ H and only weakly dependent on the mass of the perturber. We generalize the problem to include the effects of vertical tidal forcing in a vertically isothermal atmosphere with γ > 1, and calculate the vertical flux of angular momentum of acoustic waves with m < r/H. The

  13. Smoothed Particle Hydrodynamics: Numerical Techniques and Applications to Accretion Disks in Close Binaries.

    NASA Astrophysics Data System (ADS)

    Simpson, James Charles

    1995-01-01

    Numerical techniques for the method of smoothed particle hydrodynamics (SPH) are described for three dimensional systems in the absence of self-gravity. These include a method for locating neighboring particles using an Eulerian grid that conserves memory by partitioning the computational space into manageable layers. Further savings in memory and computational time are achieved using interparticle distances that are discretized with respect to integral increments of the smoothing length. We also present a time integration algorithm using multiple time steps which guarantees that all particles maintain phase space synchronicity to at least first order accuracy with respect to the time steps. These techniques are used to compare currently available SPH artificial viscosities in accretion disks in low mass ratio systems (q = {M_2 }over{M_1} = 0.02) using the ideal gas law and low adiabatic gamma (gamma = 1.01): radiation effects and magnetic fields are excluded. The standard artificial viscosity given by Monaghan (1992) consistently gives results which agree qualitatively with the Shukura-Sunyaev alpha -disk model. The radial temperature profile is overestimated by a factor of about 10, indicating that radiation effects must be included for a complete model. This viscosity is also used to compare accretion disks in systems with the four different extreme mass ratios: q = 0.01, 0.02, 0.03, and 0.05. These simulated disks are stable with high mass transfer rates. The outer disk edges do not extend into the regions of the low order corotation resonances, consequently no precession is observed. The disks are slightly asymmetric with spiral density waves that appear stationary in the corotating frame. These may be responsible for the double humped pulses in the light curves of these systems. Finally, the free expansion of an ideal gas into a vacuum is simulated using elastic collisions between SPH particles and impenetrable flat surfaces. After _sp{~}<100 interparticle

  14. ON THE DOUBLE NATURED SOLUTIONS OF THE TWO-TEMPERATURE EXTERNAL SOFT PHOTON COMPTONIZED ACCRETION DISKS

    SciTech Connect

    Meirelles Filho, Cesar

    2009-08-01

    We have analyzed pair production in the innermost region of a two-temperature external soft photon Comptonized accretion disk. We have shown that, if the viscosity parameter is greater than a critical value {alpha}{sub c}, the solution to the disk equation is double valued: one, advection dominated, and the other, radiation dominated. When {alpha} {<=} {alpha}{sub c}, the accretion rate has to satisfy m-dot{sub 1}{<=}m-dot{<=}m-dot{sub c} in order to have two steady-state solutions. It is shown that these critical parameters m-dot{sub 1}, m-dot{sub c} are functions of r, {alpha}, and {theta}{sub e}, and {alpha}{sub c} is a function of r and {theta}{sub e}. Depending on the combination of the parameters, the advection-dominated solution may not be physically consistent. It is also shown that the electronic temperature is maximum at the onset of the thermal instability, from which results this inner region. These solutions are stable against perturbations in the electron temperature and in the density of pairs.

  15. A SUBSTANTIAL DUST DISK SURROUNDING AN ACTIVELY ACCRETING FIRST-ASCENT GIANT STAR

    SciTech Connect

    Melis, C.; Zuckerman, B.; Rhee, Joseph H.; Metchev, Stanimir; Song, Inseok

    2009-05-10

    We report identification of the first unambiguous example of what appears to be a new class of first-ascent giant stars that are actively accreting gas and dust and that are surrounded by substantial dusty disks. These old stars, who are nearing the end of their lives, are experiencing a rebirth into characteristics typically associated with newborn stars. The F2-type first-ascent giant star TYC 4144 329 2 is in a wide separation binary system with an otherwise normal G8 IV star, TYC 4144 329 1. From Keck near-infrared imaging and high-resolution spectroscopy, we are able to determine that these two stars are {approx}1 Gyr old and reside at a distance of {approx}550 pc. One possible explanation for the origin of the accreting material is common-envelope interaction with a low-mass stellar or substellar companion. The gaseous and dusty material around TYC 4144 329 2, as it is similar to the primordial disks observed around young classical T Tauri stars, could potentially give rise to a new generation of planets and/or planetesimals.

  16. V3885 Sagittarius: A Comparison With a Range of Standard Model Accretion Disks

    NASA Technical Reports Server (NTRS)

    Linnell, Albert P.; Godon, Patrick; Hubeny, Ivan; Sion, Edward M; Szkody, Paula; Barrett, Paul E.

    2009-01-01

    A chi-squared analysis of standard model accretion disk synthetic spectrum fits to combined Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph spectra of V3885 Sagittarius, on an absolute flux basis, selects a model that accurately represents the observed spectral energy distribution. Calculation of the synthetic spectrum requires the following system parameters. The cataclysmic variable secondary star period-mass relation calibrated by Knigge in 2006 and 2007 sets the secondary component mass. A mean white dwarf (WD) mass from the same study, which is consistent with an observationally determined mass ratio, sets the adopted WD mass of 0.7M(solar mass), and the WD radius follows from standard theoretical models. The adopted inclination, i = 65 deg, is a literature consensus, and is subsequently supported by chi-squared analysis. The mass transfer rate is the remaining parameter to set the accretion disk T(sub eff) profile, and the Hipparcos parallax constrains that parameter to mas transfer = (5.0 +/- 2.0) x 10(exp -9) M(solar mass)/yr by a comparison with observed spectra. The fit to the observed spectra adopts the contribution of a 57,000 +/- 5000 K WD. The model thus provides realistic constraints on mass transfer and T(sub eff) for a large mass transfer system above the period gap.

  17. Angular momentum transport and particle acceleration during magnetorotational instability in a kinetic accretion disk.

    PubMed

    Hoshino, Masahiro

    2015-02-13

    Angular momentum transport and particle acceleration during the magnetorotational instability (MRI) in a collisionless accretion disk are investigated using three-dimensional particle-in-cell simulation. We show that the kinetic MRI can provide not only high-energy particle acceleration but also enhancement of angular momentum transport. We find that the plasma pressure anisotropy inside the channel flow with p(∥)>p(⊥) induced by active magnetic reconnection suppresses the onset of subsequent reconnection, which, in turn, leads to high-magnetic-field saturation and enhancement of the Maxwell stress tensor of angular momentum transport. Meanwhile, during the quiescent stage of reconnection, the plasma isotropization progresses in the channel flow and the anisotropic plasma with p(⊥)>p(∥) due to the dynamo action of MRI outside the channel flow contribute to rapid reconnection and strong particle acceleration. This efficient particle acceleration and enhanced angular momentum transport in a collisionless accretion disk may explain the origin of high-energy particles observed around massive black holes.

  18. Disk-accreting magnetic neutron stars as high-energy particle accelerators

    NASA Technical Reports Server (NTRS)

    Hamilton, Russell J.; Lamb, Frederick K.; Miller, M. Coleman

    1994-01-01

    Interaction of an accretion disk with the magnetic field of a neutron star produces large electromotive forces, which drive large conduction currents in the disk-magnetosphere-star circuit. Here we argue that such large conduction currents will cause microscopic and macroscopic instabilities in the magnetosphere. If the minimum plasma density in the magnetosphere is relatively low is less than or aproximately 10(exp 9)/cu cm, current-driven micro-instabilities may cause relativistic double layers to form, producing voltage differences in excess of 10(exp 12) V and accelerating charged particles to very high energies. If instead the plasma density is higher (is greater than or approximately = 10(exp 9)/cu cm, twisting of the stellar magnetic field is likely to cause magnetic field reconnection. This reconnection will be relativistic, accelerating plasma in the magnetosphere to relativistic speeds and a small fraction of particles to very high energies. Interaction of these high-energy particles with X-rays, gamma-rays, and accreting plasma may produce detectable high-energy radiation.

  19. Angular momentum transport and particle acceleration during magnetorotational instability in a kinetic accretion disk.

    PubMed

    Hoshino, Masahiro

    2015-02-13

    Angular momentum transport and particle acceleration during the magnetorotational instability (MRI) in a collisionless accretion disk are investigated using three-dimensional particle-in-cell simulation. We show that the kinetic MRI can provide not only high-energy particle acceleration but also enhancement of angular momentum transport. We find that the plasma pressure anisotropy inside the channel flow with p(∥)>p(⊥) induced by active magnetic reconnection suppresses the onset of subsequent reconnection, which, in turn, leads to high-magnetic-field saturation and enhancement of the Maxwell stress tensor of angular momentum transport. Meanwhile, during the quiescent stage of reconnection, the plasma isotropization progresses in the channel flow and the anisotropic plasma with p(⊥)>p(∥) due to the dynamo action of MRI outside the channel flow contribute to rapid reconnection and strong particle acceleration. This efficient particle acceleration and enhanced angular momentum transport in a collisionless accretion disk may explain the origin of high-energy particles observed around massive black holes. PMID:25723200

  20. CONNECTION BETWEEN THE ACCRETION DISK AND JET IN THE RADIO GALAXY 3C 111

    SciTech Connect

    Chatterjee, Ritaban; Marscher, Alan P.; Jorstad, Svetlana G.; Harrison, Brandon; Agudo, Ivan; Taylor, Brian W.; Markowitz, Alex; Rivers, Elizabeth; Rothschild, Richard E.; McHardy, Ian M.; Aller, Margo F.; Aller, Hugh D.; Laehteenmaeki, Anne; Tornikoski, Merja; Gomez, Jose L.; Gurwell, Mark

    2011-06-10

    We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 111 between 2004 and 2010 at X-ray (2.4-10 keV), optical (R band), and radio (14.5, 37, and 230 GHz) wave bands, as well as multi-epoch imaging with the Very Long Baseline Array (VLBA) at 43 GHz. Over the six years of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. This shows a clear connection between the radiative state near the black hole, where the X-rays are produced, and events in the jet. The X-ray continuum flux and Fe line intensity are strongly correlated, with a time lag shorter than 90 days and consistent with zero. This implies that the Fe line is generated within 90 lt-day of the source of the X-ray continuum. The power spectral density function of X-ray variations contains a break, with a steeper slope at shorter timescales. The break timescale of 13{sup +12}{sub -6} days is commensurate with scaling according to the mass of the central black hole based on observations of Seyfert galaxies and black hole X-ray binaries (BHXRBs). The data are consistent with the standard paradigm, in which the X-rays are predominantly produced by inverse Compton scattering of thermal optical/UV seed photons from the accretion disk by a distribution of hot electrons-the corona-situated near the disk. Most of the optical emission is generated in the accretion disk due to reprocessing of the X-ray emission. The relationships that we have uncovered between the accretion disk and the jet in 3C 111, as well as in the Fanaroff-Riley class I radio galaxy 3C 120 in a previous paper, support the paradigm that active galactic nuclei and Galactic BHXRBs are fundamentally similar, with characteristic time and size scales proportional to the mass of the central black hole.

  1. Stellar parameters and accretion rate of the transition disk star HD 142527 from X-shooter

    SciTech Connect

    Mendigutía, I.; Fairlamb, J.; Oudmaijer, R. D.; Montesinos, B.; Najita, J. R.; Brittain, S. D.; Van den Ancker, M. E.

    2014-07-20

    HD 142527 is a young pre-main-sequence star with properties indicative of the presence of a giant planet and/or a low-mass stellar companion. We have analyzed an X-Shooter/Very Large Telescope spectrum to provide accurate stellar parameters and accretion rate. The analysis of the spectrum, together with constraints provided by the spectral energy distribution fitting, the distance to the star (140 ± 20 pc), and the use of evolutionary tracks and isochrones, led to the following set of parameters: T{sub eff} = 6550 ± 100 K, log g = 3.75 ± 0.10, L{sub *}/L{sub ☉} = 16.3 ± 4.5, M{sub *}/M{sub ☉} = 2.0 ± 0.3, and an age of 5.0 ± 1.5 Myr. This stellar age provides further constraints to the mass of the possible companion estimated by Biller et al., being between 0.20 and 0.35 M{sub ☉}. Stellar accretion rates obtained from UV Balmer excess modeling and optical photospheric line veiling, and from the correlations with several emission lines spanning from the UV to the near-IR, are consistent with each other. The mean value from all previous tracers is 2 (±1) × 10{sup –7} M{sub ☉} yr{sup –1}, which is within the upper limit gas flow rate from the outer to the inner disk recently provided by Cassasus et al.. This suggests that almost all gas transferred between both components of the disk is not trapped by the possible planet(s) in between but fall onto the central star, although it is discussed how the gap flow rate could be larger than previously suggested. In addition, we provide evidence showing that the stellar accretion rate of HD 142527 has increased by a factor ∼7 on a timescale of 2 to 5 yr.

  2. TURBULENCE AND STEADY FLOWS IN THREE-DIMENSIONAL GLOBAL STRATIFIED MAGNETOHYDRODYNAMIC SIMULATIONS OF ACCRETION DISKS

    SciTech Connect

    Flock, M.; Dzyurkevich, N.; Klahr, H.; Turner, N. J.; Henning, Th.

    2011-07-10

    We present full 2{pi} global three-dimensional stratified magnetohydrodynamic (MHD) simulations of accretion disks. We interpret our results in the context of protoplanetary disks. We investigate the turbulence driven by the magnetorotational instability (MRI) using the PLUTO Godunov code in spherical coordinates with the accurate and robust HLLD Riemann solver. We follow the turbulence for more than 1500 orbits at the innermost radius of the domain to measure the overall strength of turbulent motions and the detailed accretion flow pattern. We find that regions within two scale heights of the midplane have a turbulent Mach number of about 0.1 and a magnetic pressure two to three orders of magnitude less than the gas pressure, while in those outside three scale heights the magnetic pressure equals or exceeds the gas pressure and the turbulence is transonic, leading to large density fluctuations. The strongest large-scale density disturbances are spiral density waves, and the strongest of these waves has m = 5. No clear meridional circulation appears in the calculations because fluctuating radial pressure gradients lead to changes in the orbital frequency, comparable in importance to the stress gradients that drive the meridional flows in viscous models. The net mass flow rate is well reproduced by a viscous model using the mean stress distribution taken from the MHD calculation. The strength of the mean turbulent magnetic field is inversely proportional to the radius, so the fields are approximately force-free on the largest scales. Consequently, the accretion stress falls off as the inverse square of the radius.

  3. Accretion Kinematics through the Warped Transition Disk in HD142527 from Resolved CO(6-5) Observations

    NASA Astrophysics Data System (ADS)

    Casassus, S.; Marino, S.; Pérez, S.; Roman, P.; Dunhill, A.; Armitage, P. J.; Cuadra, J.; Wootten, A.; van der Plas, G.; Cieza, L.; Moral, Victor; Christiaens, V.; Montesinos, Matías

    2015-10-01

    The finding of residual gas in the large central cavity of the HD 142527 disk motivates questions regarding the origin of its non-Keplerian kinematics and possible connections with planet formation. We aim to understand the physical structure that underlies the intra-cavity gaseous flows, guided by new molecular-line data in CO(6-5) with unprecedented angular resolutions. Given the warped structure inferred from the identification of scattered-light shadows cast on the outer disk, the kinematics are consistent, to first order, with axisymmetric accretion onto the inner disk occurring at all azimuths. A steady-state accretion profile, fixed at the stellar accretion rate, explains the depth of the cavity as traced in CO isotopologues. The abrupt warp and evidence for near free-fall radial flows in HD 142527 resemble theoretical models for disk tearing, which could be driven by the reported low-mass companion, whose orbit may be contained in the plane of the inner disk. The companion’s high inclination with respect to the massive outer disk could drive Kozai oscillations over long timescales; high-eccentricity periods may perhaps account for the large cavity. While shadowing by the tilted disk could imprint an azimuthal modulation in the molecular-line maps, further observations are required to ascertain the significance of azimuthal structure in the density field inside the cavity of HD 142527.

  4. ACCRETION KINEMATICS THROUGH THE WARPED TRANSITION DISK IN HD 142527 FROM RESOLVED CO(6–5) OBSERVATIONS

    SciTech Connect

    Casassus, S.; Marino, S.; Pérez, S.; Plas, G. van der; Christiaens, V.; Montesinos, Matías; Roman, P.; Dunhill, A.; Cuadra, J.; Cieza, L.; Moral, Victor; Armitage, P. J.; Wootten, A.

    2015-10-01

    The finding of residual gas in the large central cavity of the HD 142527 disk motivates questions regarding the origin of its non-Keplerian kinematics and possible connections with planet formation. We aim to understand the physical structure that underlies the intra-cavity gaseous flows, guided by new molecular-line data in CO(6–5) with unprecedented angular resolutions. Given the warped structure inferred from the identification of scattered-light shadows cast on the outer disk, the kinematics are consistent, to first order, with axisymmetric accretion onto the inner disk occurring at all azimuths. A steady-state accretion profile, fixed at the stellar accretion rate, explains the depth of the cavity as traced in CO isotopologues. The abrupt warp and evidence for near free-fall radial flows in HD 142527 resemble theoretical models for disk tearing, which could be driven by the reported low-mass companion, whose orbit may be contained in the plane of the inner disk. The companion’s high inclination with respect to the massive outer disk could drive Kozai oscillations over long timescales; high-eccentricity periods may perhaps account for the large cavity. While shadowing by the tilted disk could imprint an azimuthal modulation in the molecular-line maps, further observations are required to ascertain the significance of azimuthal structure in the density field inside the cavity of HD 142527.

  5. Numerical experiments in galactic disks: Gravitational instability, stochastic accretion, and galactic winds

    NASA Astrophysics Data System (ADS)

    Forbes, John C.

    Using 0D, 1D, and 3D models of galaxies, I explore different problems in galaxy evolution most suited to each technique. In the simplest case, a galaxy is described by a few numbers integrated via coupled ordinary differential equations. By allowing the galaxies to respond to a stochastic accretion rate, I show a natural way of generating the finite scatter observed in several galaxy scaling relations: the correlation between a galaxy's stellar mass and its star formation rate or metallicity. By comparing this simple model to observations, we constrain the process by which gas accretes onto galaxies, which must occur, but is essentially impossible to observe directly. Adding an additional dimension to the models, we explore the structure of galactic disks as a function of radius. We find that turbulence driven by gravitational instability in the disks and the resulting migration of gas can explain a wide variety of phenomena, including the age-velocity dispersion correlation of stars in the solar neighborhood, the central quenching star formation in disk galaxies, rings of star formation, and the observed radial profile of gas column densities. Finally, we run a set of fully three-dimensional galaxy simulations to try to understand what physics is responsible for basic properties of galaxies, including the rate at which they form stars, and the rate at which they eject mass in large-scale winds. We find that supernovae are capable of driving moderate metal-enhanced winds, but surprisingly they have very little effect on the star formation rates of dwarf galaxies. Instead, ordinary photoelectric heating dominates the star formation law in low-mass galaxies.

  6. DETERMINATION OF CENTRAL ENGINE POSITION AND ACCRETION DISK STRUCTURE IN NGC 4261 BY CORE SHIFT MEASUREMENTS

    SciTech Connect

    Haga, Takafumi; Doi, Akihiro; Murata, Yasuhiro; Sudou, Hiroshi; Kameno, Seiji; Hada, Kazuhiro

    2015-07-01

    We report multifrequency phase-referenced observations of the nearby radio galaxy NGC 4261, which has prominent two-sided jets, using the Very Long Baseline Array at 1.4–43 GHz. We measured radio core positions showing observing frequency dependences (known as “core shift”) in both approaching jets and counterjets. The limit of the core position as the frequency approaches infinity, which suggests a jet base, is separated by 82 ± 1