Thermal instability and photoionized x-ray reflection in accretion disks

We study the illumination of accretion disks in the vicinity of compact objects by an overlying X-ray source. Our approach differs from previous works of the subject in that we relax the simplifying assumption of constant gas density used in these studies; instead we determine the density from hydrostatic balance which is solved simultaneously with the ionization balance and the radiative transfer in a plane-parallel geometry. We find that the self-consistent density determination makes evident the presence of a thermal ionization instability. The main effect of this instability is to prevent the illuminated gas from attaining temperatures at which the gas is unstable to thermal perturbations. In sharp contrast to the constant density calculations that predict a continuous and rather smooth variation of the gas temperature in the illuminated material, we find that the temperature profile consists of several well defined thermally stable layers. In particular, the uppermost layers of the X-ray illuminated gas are found to be almost completely ionized and at the local Compton temperature (∼107–108 K); at larger depths, the gas temperature drops abruptly to form a thin layer with T∼106 K, while at yet larger depths it decreases sharply to the disk effective temperature. The results of our self-consistent calculations are both quantitatively and qualitatively different from those obtained using the constant density assumption. We believe that usage of the latter can be completely misleading in attempts to understand the accretion disk structure from observations of iron lines and the reflection component.