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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint B/97

Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects II. Application to Cygnus X-1

J. B. Dove(1,2), J. Wilms(3,1), M. Maisack(3), 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 apply our self-consistent accretion disk corona (ADC) model, with two different geometries, to the broad-band X-ray spectrum of the black hole candidate Cygnus X-1. As shown in a companion paper (Dove, Wilms, and Begelman, 1997), models where the Comptonizing medium is a slab surrounding the cold accretion disk cannot have a temperature higher than about 120 keV for optical depths greater than 0.2, resulting in spectra that are much softer than the observed 10-30 keV spectrum of Cyg X-1. In addition, the slab geometry models predict a substantial ``soft excess'' at low energies, a feature not observed for Cyg X-1, and Fe Kalpha fluorescence lines that are stronger than observed. Previous Comptonization models in the literature invoke a slab geometry with the optical depth greater than about 0.3 and the coronal temperature of about 150 keV, but they are not self-consistent. Therefore, ADC models with a slab geometry are not appropriate for explaining the X-ray spectrum of Cyg X-1. Models with a spherical corona and an exterior disk, however, predict much higher self-consistent coronal temperatures than the slab geometry models. The higher coronal temperatures are due to the lower amount of reprocessing of coronal radiation in the accretion disk, giving rise to a lower Compton cooling rate. Therefore, for the sphere+disk geometry, the predicted spectrum can be hard enough to describe the observed X-ray continuum of Cyg X-1 while predicting Fe fluorescence lines having an equivalent width of about 40 eV. Our best-fit parameter values for the sphere+disk geometry are an optical depth of about 1.5 and a coronal temperature of about 90 keV.

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

Paper (93k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de)
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