Observational Signatures of X-Ray-irradiated Accretion Disks

Reflection of X-rays from cool material around a black hole is one of the few observational diagnostics of the accretion flow geometry. Models of this reflected spectrum generally assume that the accretion disk can be characterized by material in a single ionization state. However, several authors have recently stressed the importance of the classic ionization instability for X-ray-irradiated gas in hydrostatic balance. This instability leads to a discontinuous transition in the vertical structure of the disk, resulting in a hot ionized skin above much cooler material. If the Compton temperature of the skin is high, then even iron is completely ionized, and the skin does not produce any spectral features. These new models, where the ionization structure of the disk is calculated self-consistently, require an excessive amount of computing power and so are difficult to use in directly fitting observed X-ray spectra. Instead, we invert the problem by simulating X-ray spectra produced by the new reflection models and then fit these with the old, single-zone reflection models, to assess the extent to which the derived accretion geometry depends on the reflection model used. We find that the single-zone ionization models can severely underestimate the covering fraction of the "cold" material as seen from the X-ray source if the optical depth in the ionized skin is of order unity and that this can produce an apparent correlation between the covering fraction and the X-ray spectral index similar in nature to that reported by Zdziarski, Lubiński, and Smith.