A two-temperature accretion disk model for Cygnus X-1 - Structure and spectrum

A model for Cygnus X-1, involving an accretion disk around a black hole, is presented which can explain the observed X-ray spectrum from 8 to 500 keV. In particular, a detailed model is constructed for the structure of an accretion disk whose inner region is considerably hotter and geometrically thicker than previous disk models. The inner region of the disk is optically thin to absorption, is gas-pressure dominated, and yields, from first principles, electron temperatures of 1 billion K and ion temperatures 3 to 300 times hotter. The spectrum above 8 keV is produced by inverse Compton scattering of soft X-ray photons in the two-temperature inner region of the disk. This spectrum is computed by numerical integration of the Kompane'ets equation, modified to account for escape of photons from a region of finite (order unity) electron-scattering optical depth.