Pressure Ionization Instability: Connection between Seyferts and GBHCs

Abstract

Spectrum of Seyfert 1 Galaxies (S1G hereafter) 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. However, recent work has shown that reprocessing features make the patchy corona-disk model (PCD model: the best current explanation of S1G spectrum) problematic for GBHCs. To address the similarities and differences in spectrum of Seyferts and GBHCs, we consider the structure of the ionized X-ray skin near an active magnetic flare. We show that the X-ray skin is subject to a thermal instability, similar in nature to the well known ionization instability of quasar emission line regions. 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 ($\sim $ few keV). We show that this accounts for the difference in spectrum of GBHCs and S1G. In addition, same instability, applied to S1G, leads to the X-ray skin temperature $T\sim 1-3\times 10^5$ Kelvin, which then may explain the observed spectral shape of BBB.