摘要:

I review recent numerical studies of accretion disks, focusing on measurements of the turbulent shear stress, or &agr;, in the shearing box model. I conclude with a list of astronomically relevant open questions that can be settled via future numerical experiments. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55861

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The Fourier frequency dependent hard X-ray lag, first discovered from the analysis of aperiodic variability of the light curves of the black hole candidate Cygnus X-1, turns out to be a property shared by several other accreting compact sources. We show that the lag can be explained in terms of Comptonization process in coronae of hot electrons with inhomogeneous density distributions. The density profile of a corona, like the optical depth and electron temperature, can significantly affect the Comptonization energy spectrum from it. This means, by fitting the energy spectrum alone, it is not possible to uniquely determine the optical depth and temperature of the Comptonization cloud if its density profile is unknown. The hard X-ray time lag is sensitive to the density distribution of the scattering corona. Thus simultaneous analysis of the spectral and temporal X-ray data will allow us to probe the density structure of the Comptonizing corona and thereby the dynamics of mass accretion onto the compact object. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55881

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present model light curves, Power Spectral Densities (PSD) and time lags of accreting Black Hole Candidates (BHC) based on a recent model of these sources. According to our model the observed variability is due, to a large extent, to the stochastic nature of Comptonization, the basic process responsible also for the emission of high energy radiation. Our additional assumption is that the Comptonization process takes place in an extended but non-uniform atmosphere around the compact object. Our model reproduces the observed light curves well, in that it provides a good fit to the PSDs and the overall light curve morphology, indicating, in accordance with observation, that most of the power resides at time scales ≳ a few seconds while at the same time one can distinguish shots of a few msec in duration. We suggest that refinement of this type of model along with spectral and phase lag information can be used to probe the structure of this class of high energy sources. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55882

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The time-dependent vertical structure of a cold accretion disk is studied semi-analytically and a stability criterion has been derived. Turbulent heat transfer is able to stabilize thermally if the Prandtl number as well as the viscosity &agr; is lower than, say, 0.1. However, for very low values of these parameters numerical simulations reveal the onset of over-stability. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55883

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We use a bi-grey two-stream model to calculate the vertical structure of X-ray irradiated accretion discs, and to investigate their S-curves and disc instability. The results are applied to Black Hole Soft X-ray Transients (SXTs). ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55884

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present the results of detailed numerical calculations of the transport of radiation in spherical accretion flows onto compact objects. We show that Comptonization by the electron bulk motion should not be neglected in computing the X-ray spectra of accreting compact objects and demonstrate the importance of retaining the terms in the radiative transfer equation that are at least second-order in the electron bulk velocity. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55885

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We discuss the spectral states of Galactic Black Holes Candidates (GBHC) within the context of a magnetically structured corona model, in which magnetic flares result from reconnection of flux tubes rising from the underlying accretion disk due to magnetic buoyancy. Using a simple geometrical representation, we have shown that the hard X-ray state in GBHC can be produced by flares locatedhighabove the underlying disk via Comptonization of the cyclo-synchrotron radiation. The soft state is due to flaring of active regionscloseto the disk. The soft blackbody component is the result of re-radiation from the disk of X-rays and the hard tail is produced via Comptonization of such a soft component. By varying the typical scale size and scale height of the dissipative regions, we have qualitatively reproduced the spectra for all of the five states observed in GX339-4. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55862

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We aim to employ a fully self-consistent atmosphere code to determine surface density, &Sgr;, effective temperature,Teff,and microturbulence,Vturb,from emission line spectra of quiescent accretion discs. We have built a grid of solar-abundance, LTE, isothermal disc atmosphere models and investigate the sensitivity of the optical spectrum toTeff,&Sgr; andVturb.The results of fitting the grid to a synthetic quiescent disc spectrum and the spectrum of the dwarf nova SS Cyg are presented. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55863

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We compute the relation between the radius at the edge of the self-gravitating regime, the central mass, the accretion rate, and the &agr;-viscosity parameter, for accretion disks powering Active Galactic Nuclei. Differences with other models are briefly discussed. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55886

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The observability of the tidal disruption of stars by massive black holes in galactic nuclei is examined. At least three phases following a tidal disruption event may have spectacular and observable consequences: (1) a flare produced by the initial fast disk accretion of the disrupted material which may have absolute magnitudes in excess of −19 in V and −21 in U, (2) a long lasting,∼102–103year phase in which the mass to light ratio of the black hole remains below 1, and (3) a nearly constant level of nebular line-emission from recombining highly ionized species such as HeII. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.55887

出版商:AIP    

年代:1998

数据来源: AIP