A two-phase model for the X-ray emission from Seyfert galaxies

Abstract

A two-phase accretion disk model is considered. A substantial fraction, f of the gravitational power is assumed to be dissipated via buoyancy and reconnection of magnetic fields in a hot tenuous "corona" surrounding the main body of the disk. The main cooling mechanism of the hot layer is Comptonization of soft photons, thermally produced in the underlying cold phase. Hard Comptonized photons are backscattered into the thick phase, where they are in part reprocessed into the soft blackbody power and in part reflected. Coupled thermal balance equations for the two phases yield the temperature of the hot phase and the slope of the Comptonized component self-consistently as a function of f and τ, the optical depth of the hot phase. We find that for f~1, and τ < 1, the temperature of the hot phase adjusts so as to maintain α~1. For small τ the temperature of the hot phase is sufficient for production of electron-positron pairs to be important. Pair production contributes to the optical depth and limits the maximum temperature allowed. Monte Carlo simulations with parameters satisfying the balance equations show that, due to the asymmetry between forward and backward scattering, the transmitted spectrum in the 2-30 keV range is significantly flatter than derived from angle averaged formulae and the photon flux available for reprocessing in the thick layer is larger than 50%. The model accounts in a self-consistent way for the average power- law index and reflection component observed in the X-ray emission of Seyfert galaxies.