E. Meyer-Hofmeister

Theory of accretion disks--2

Preface. Protoplanetary disks S.V.W. Beckwith. Magnetospheric accretion and winds from pre-main sequence disks L. Hartmann. Forbidden line diagnostics of disk winds in YSOs R.E. Pudritz, R. Ouyed. Magnetized accretion disks and the origin of bipolar flows A. Konigl. Wind magnetospheres of protostellar accretion disks Ch. Fendt, M. Camenzind. Accretion disks in cataclysmic variable stars: Recent observational developments K. Horne. Disk instability model for SU UMa stars: SU UMa/WZ Sge connection Y. Osaki. Eccentric disk instabilities and their relation to superhump binaries S.H. Lubow. On the sequencing of long and short outbursts in dwarf novae J.K. Cannizzo. Dynamics of dwarf novae accretion discs R. Whitehurst. The viscous evolution of elliptical accretion discs D. Syer, C. Clarke. Probing the kinematics of disk winds F.V. Hessman, J. Eisloffel. Coronal winds producing the tilted and twisted shape of the accretion disk in Her X-1 S. Horn, F. Meyer. Recurrence time for dwarf nova outbursts: Dependence on mass transfer rate S. Ichikawa, Y. Osaki. Circulation, pulsation and convection in accretion disks W. Kley. Instability in a braking disk: Her X-1 in complete-off state F. Meyer, E. Meyer-Hofmeister. Temperature profiles of accretion disks in X-ray binaries S. Mineshige, F. Honma, A. Hirano, S. Kitamoto, T. Yamada, J. Fukue. An empirical approach to accretion disks in AGN S. Collin-Souffrin. Periodic X-ray variability of the Seyfert galaxy NGC 6814 M.A. Abramowicz, G. Bao, V. Karas, A. Lanza. Atmospheres and thermal continua of AGN disks G.A. Shields, H.H. Coleman. Star-disk interaction in quasars and AGNs D.N.C. Lin, P. Artymowicz, J.Wampler. How to tyransport angular momentum in the inner 150 pc of galaxies W.J. Duschl, S. von Linden, P.L. Biermann. The energy budget in the irradiation model of quasars M.-H.D. Ulrich. X-ray illumination of AGN disks H. Netzer. Pulsational instability of relativistic accretion disks and the periodic X-ray time variability of NGC 6814 F. Honma, R. Matsumoto, S. Kato. Large-scale dynamics of molecular clouds in the interstellar medium of disk galaxies S. von Linden, H. Lesch, F. Combes. A self-consistent accretion disk-coronal model for the active galacticnuclei K. Nakamura, Y. Osaki. Simulations with smoothed particles confirm stationary shocks in accretion flows onto Black Holes D. Molteni, S. Chakrabarti. A single variability model for NGC 6814 and NGC 5548 F. Wallinder. Accretion disk boundary layers R. Narayan, R. Popham. Measuring the radial extent of the disk-star boundary layer in T Tauri stars W.J. Duschl, W.M. Tscharnuter, C. Bertout, J. Bouvier. Magnetic interaction between accretion disks and supermassive Black Holes in AGN M. Camenzind. On the dynamics of warped accretion disks J.C.B. Papaloizou, D.N.C. Lin. Slim accretion discs J.P. Lasota. A two dimensional steady state configuration of a thick accretion disk E. Szuszkiewicz, J. Paploizou. Recent work on angular momentum transport in accretion discs J.E. Pringle. Tidal excitation of spiral waves in accretion discs G.J. Savonije, J.C.B. Paploizou, D.N.C. Lin. Magnetic discs A.R. King, G.A. Wynn. A magnetic turbulence model in disks S. Kato. Electron-positron pairs in accretion disks M. Kusunose, S. Mineshige. Intriguing morphologies, jets and disks in planetary nebulae M. Livio.<

THE EXISTENCE OF INNER COOL DISKS IN THE LOW/HARD STATE OF ACCRETING BLACK HOLES

The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low/hard state. A description of a simple model for the exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low/hard state of black hole transient systems for a range of luminosities that depends on the magnitude of the viscosity parameter. For α ~ 0.1-0.4, an inner disk can exist for luminosities in the range ~(0.001-0.02)LEdd. The model is applied to the X-ray observations of the black hole candidate sources GX 339-4 and SWIFT J1753.5-0127 in their low/hard state. It is found that Compton cooling is important in the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low-luminosity active galactic nuclei.

Two-temperature coronal flow above a thin disk

We extended the disk corona model to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate, and hence the fraction of accretion energy released in the corona, depend strongly on the rate of incoming mass flow from the outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation, and a weaker corona. We also found a strong dependence on the viscosity, with higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate, our results become independent of the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies, in agreement with observations. For luminous active galactic nuclei, strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.

Re-condensation from an ADAF into an inner disk: the intermediate state of black hole accretion?

Context. Accretion onto galactic and supermassive black holes occurs in different modes, which are documented in hard and soft spectral states, commonly attributed to an advection-dominated flow (ADAF) inside a truncated disk and standard disk accretion, respectively. At the times of spectral transition an intermediate state is observed, for which the accretion flow pattern is still unclear. Aims. We analyze the geometry of the accretion flow when the mass flow rate in the disk decreases (soft/hard transition) and evaporation of gas into the coronal flow leads to disk truncation. Methods. We evaluate the physics of an advection-dominated flow affected by thermal conduction to a cool accretion disk underneath. Results. We find re-condensation of gas from the ADAF into the underlying inner disk at distances from the black hole and at rates, that depend on the properties of the hot ADAF and vary with the mass accretion rate. This sustains an inner disk for longer than a viscous decay time after the spectral transition occurred, in accordance with the spectra that indicate cool gas in the neighborhood of the accreting black hole. The model allows us to understand why Cyg X-1 does not show hysteresis in the spectral state transition luminosity that is commonly observed for X-ray transient sources. Conclusions. Our results shed new light on the complex mass flow pattern during spectral state transition.

Hysteresis in spectral state transitions - A Challenge for theoretical modeling

Many low-mass X-ray binaries show both hard and soft spectral states. For several sources the transitions between these states have been observed, mostly from the soft to the hard state during a luminosity decrease. In a few cases also the transition from the hard to the soft state was observed, coincident with an increase of the luminosity. Surprisingly this luminosity was not the same as the one during a following change back to the hard state. The values differed by a factor of about 3 to 5. We present a model for this hysteresis in the light curves of low-mass X-ray binaries (sources with neutron stars or black holes). We show that the different amount of Compton cooling or heating acting on the accretion disk corona at the time of the transition causes this switch in the accretion mode at different mass accretion rates and therefore different luminosities. The inner disk during the soft state provides a certain amount of Compton cooling which is either not present or much less if the inner region is filled with a hot advection-dominated accretion flow (ADAF) that radiates a hard spectrum.

On the Properties of Inner Cool Disks in the Hard State of Black Hole X-Ray Transient Systems

We investigate the formation of cool disks in the innermost regions of black hole X-ray transient systems in the low/hard state. Taking into account the combined cooling associated with the Compton and conductive energy transport processes in a corona, we describe the radial structure of a disk for a range of mass accretion rates. The mass flow in an optically thick inner region can be maintained by the condensation of matter from a corona, with the disk temperature and luminosity varying continuously as a function of the accretion rate. Although such a disk component can be present, the contribution of the optically thick disk component to the total luminosity can be small, since the mass flow due to condensation in the optically thick disk underlying the corona can be significantly less than the mass flow in the corona. The model is applied to the observations of the low/hard state of the black hole source GX 339-4 at luminosities of similar to 0.01L(Edd) and is able to explain the temperature of the thermal component at the observed luminosities. Since conductive cooling dominates Compton cooling at low mass accretion rates, the luminosity corresponding to the critical mass accretion rate above which a weak thermal disk component can be present in the low/hard state is estimated to be as low as 0.001L(Edd).

An inner disk below the ADAF: the intermediate spectral state of black hole accretion

Aims. The hard and soft spectral states of black hole accretion are understood as connected with ADAF accretion (truncated disk) and standard disk accretion, respectively. However, observations indicate the existence of cool gas in the inner region at times when the disk is already truncated outside. We try to shed light on these not yet understood intermediate states. Methods. The disk-corona model allows to understand the spectral state transitions as caused by changes of the mass flow rate in the disk and provides a picture for the accretion geometry when disk truncation starts at the time of the soft/hard transition, the formation of a gap in the disk filled by an advection-dominated flow (ADAF) at the distance where the evaporation is maximal. We study the interaction of such an ADAF with an inner thin disk below. Results. We show that, when the accretion rate is not far below the transition rate, an inner disk could exist below an ADAF, leading to an intermediate state of black hole accretion.

Spectral state transitions in low-mass x-ray binaries. The Effect of hard and soft irradiation

In neutron star and black hole X-ray binaries the transitions between the two spectral states, hard and soft, signal the change between accretion via a hot advection-dominated flow(ADAF) and disk accretion. In a few cases the hard/soft transition was observed during the rise to the nova outburst, but in most cases only the soft/hard transition during the luminosity decrease. Surprisingly the luminosities at the second transition are always lower by a factor of 3 to 5. A model for this hysteresis was presented in a preceding paper where it was shown that this switch in the accretion mode at different mass accretion rates and therefore different luminosities is caused by the different amount of Compton cooling or heating of the accretion disk corona as it is irradiated by hard or soft radiation from the central light source, respectively. We now give detailed results for the dependence on the hardness of this radiation and the radiation efficiency. We further discuss the influence of the inclination and a possible warping of the disk on the observed hysteresis.

A nova origin of the gas cloud at the galactic center

The recent discovery by Gillessen and collaborators of a cloud of gas falling towards the Galactic Center on a highly eccentric orbit, diving nearly straight into the immediate neighborhood of the central supermassive black hole, raises the important question of its origin. Several models have already been proposed. Here we suggest that a recent nova outburst has ejected a ring-like shell of gas. Viewed at high inclination, that could account for the mass, head and tail structure, and the unusually high eccentricity of the observed cloud in a natural way, even as the nova moves on an orbit quite normal for the young stars in the close neighborhood of the Galactic Center. We illustrate this by calculating orbits for the head and tail parts of the ejecta and the nova that has produced it. We briefly discuss some of the questions that this model, if true, raises about the stellar environment close to the Galactic Center.

The formation of the coronal flow/ADAF

We develop a new method to describe the accretion flow in the corona above a thin disk around a black hole in vertical and radial extent. The model is based on the same physics as the earlier one-zone model, but now modified including inflow and outflow of mass, energy and angular momentum from and towards neighboring zones. We determine the radially extended coronal flow for different mass flow rates in the cool disk resulting in the truncation of the thin disk at different distance from the black hole. Our computations show how the accretion flow gradually changes to a pure vertically extended coronal or advection-dominated accretion flow (ADAF). Different regimes of solutions are discussed. For some cases wind loss causes an essential reduction of the mass flow.