Ionization structure and Fe Kα energy for irradiated accretion discs

We study the radial ionization structure at the surface of an X-ray illuminated accretion disc. We plot the expected iron Kα line energy as a function of the Eddington ratio and of the distance of the emitting matter from the central source, for a non-rotating and a maximally rotating black hole. We compare the predicted disc line energies with those measured in an archival sample of active galactic nuclei observed with Chandra, XMM-Newton and Suzaku, and discuss whether the line energies are consistent with the radial distances inferred from reverberation studies. We also suggest using rapidly variable iron Kα lines to estimate the viscosity parameter of an accretion disc. There is a forbidden region in the line energy versus Eddington ratio plane, at low Eddington ratios, where an accretion disc cannot produce highly ionized iron Kα lines. If such emission is observed in low-Eddington-ratio sources, it is coming from either a highly ionized outflow or a blueshifted component from fast-moving neutral matter.