Institut für Astronomie und Astrophysik

Abteilung Astronomie

Preprint A/97

Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects: I. Properties of the Corona and the Spectrum of Escaping Radiation

J. B. Dove(1,2), J. Wilms(3,1), M. C. Begelman(1,2)

(1)JILA, University of Colorado, Campus Box 440, Boulder, CO 80309-0440, U.S.A.
(2)Department of Astrophysical, Planetary, and Atmospheric Sciences, University of Colorado, Boulder, Boulder, CO 80309-0391
(3)Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

accepted by Astrophysical Journal

Abstract. We present the properties of accretion disk corona (ADC) models, where the radiation field, the temperature, andthe total opacity of the corona are determined self-consistently. We use a non-linear Monte Carlo code to perform the calculations. As an example, we discuss models where the corona is situated above and below a cold accretion disk with a plane-parallel (slab) geometry, similar to the model of Haardt and Maraschi. By Comptonizing the soft radiation emitted by the accretion disk, the corona is responsible for producing the high-energy component ofthe escaping radiation. Our models include the reprocessing of radiation in the accretion disk. Here, the photons either are Compton reflected or photo-absorbed, giving rise to fluorescent line emission and thermal emission. The self-consistent coronal temperature is determined by balancing heating (due to viscous energy dissipation) with Compton cooling, determined using the fully relativistic, angle-dependent cross-sections. The total opacityis found by balancing pair productions with annihilations. We find that, for a disk temperature less than about 200 eV, these coronae are unable to have a self-consistent temperature higher than about 120 keV if the total optical depth is greater than about 0.2, regardless of the compactness parameter of the corona and the seed opacity. This limitation corresponds to the angle-averaged spectrum of escaping radiation having a photon index greater than about 1.8 within the 5 keV - 30 keV band. Finally, all models that have reprocessing features also predict a large thermal excess at lower energies. These constraints make explaining the X-ray spectra of persistent black hole candidates with ADC models very problematic.

Key words: radiation mechanisms: nonthermal - radiative transfer - X-rays: general - accretion

Paper (116k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de) Last updated 05-Mar-1998