X-rays From Magnetic Flares In Cygnus X-1: The Role Of A Transition   Layer
  The spectrum of Seyfert 1 Galaxies is very similar to that of several Galactic Black Hole Candidates (GBHCs) in their hard state, suggestive that both classes of objects have similar physical processes. While it appears that an accretion disk corona (ADC) model, where the corona sandwiches the cold accretion disk, is capable of explaining the observations of Seyfert galaxies, recent work has shown that this model is problematic for GBHCs. To address the differences in the spectra of Seyferts and GBHCs, we consider the structure of the atmosphere of the accretion disk in a region near an active magnetic flare (we refer to this region as transition layer). We show that the transition layer is subject to a thermal instability. Due to the much higher ionizing X-ray flux in GBHCs, the only stable solution for the upper layer of the accretion disk is that in which it is highly ionized and is at the Compton temperature ($kT \sim $ a few keV). Using numerical simulations for a slab geometry ADC, we show that the presence of a transition layer can significantly alter the spectrum of escaping radiation for modest optical depths. The presence of the transition layer also leads to a reduction in reprocessing features, i.e., the iron line and the X-ray reflection hump. We find that due to the thermal instability of the surface of the accretion disk, which leads to different endpoints for GBHCs and Seyfert galaxies, the X-ray spectra from these two types of objects can be explained by a single unifying ADC model. 
Sergei Nayakshin (University of Arizona, Tucson) and James B. Dove   (CASA, University of Colorado, Boulder)