1997/11/26 by Juri Poutanen, Paolo Coppi
Physics and Astronomy · #astro-ph
published as Phys.Scripta T77 (1998) 57-59 · 3 pages; 2 postscript figures; to appear in Physica Scripta in the proceedings of the Nordic Symposium "Exploiting Spectrum-X-Gamma and INTEGRAL", Copenhagen, September 14-16, 1997
arxiv created 1997/11/26 · arxiv updated 2009/12/01
Several galactic black holes show transitions between spectral states. In this paper we present a unifying radiation transfer model that can fit the spectral data in both states. We fit individual broad-band (from 1 keV up to 4 MeV) spectral data for the "hard" and "soft" states of Cyg X-1 using an emission model where a central Comptonizing corona/cloud is illuminated by the soft photon emission from a cold disk. We assume that the energy is injected to the corona by two channels: a non-thermal one that injects energetic electrons into the coronal region, and a thermal one that heats injected and ambient electrons once they cool sufficiently to form a Maxwellian distribution, i.e., we consider a hybrid thermal/non-thermal model. Using simple scaling laws for the luminosity of the cold disk, the thermal dissipation/heating rate in the corona, and the rate of energy injection from a non-thermal source, all as functions of radius of the corona, we explain the hard-to-soft transition as the result of a decrease in the size of the corona and the inner radius of the cold disk by a factor ~ 5. For the case of Cyg X-1, we show that the bolometric luminosity of the source (mass accretion rate) does not change significantly during the transition, and thus the transition is probably the result of a disk instability.