X-ray spectra from two-phase accretion disks

A model for the X-ray emission from radio-quiet active galactic nuclei (AGNs) is considered. The X-rays are produced via inverse Compton (IC) emission in a hot corona embedding a colder accretion disk. Soft thermal photons emitted by the cold layer provide the main source of cooling for the hot electrons in the corona. At the same time, about half of the high-energy photons produced in the corona are effective in heating the underlying dense layers. The local viscous dissipation within the disk is considered negligible. The energy coupling between the disk and the corona forces the electron temperature to adjust so as to maintain comparable luminosities in the soft and hard components of the emitted radiation, independently of the optical depth of the scattering medium. The coronal equilibrium temperature, as well as the resulting high-energy spectra, are computed taking into account the anisotropy of the seed photons. Production of electron-positron pairs in the corona limits the minimum possible optical depth and maximum allowed temperature. The emitted spectra consist of the intrinsic IC emission from the hot layer, plus the thermal flux and a reflected component from the cold disk. They are found to be in excellent agreement with present X-ray observations of radio-quiet AGNs.