Testing a model of variability of X-ray reprocessing features in Active Galactic Nuclei

A number of recent results from X-ray observations of Active Galactic Nuclei involving the Fe K alpha line (reduction of line variability compared to the X-ray continuum variability, the X-ray ``Baldwin effect'') were attributed to a presence of a hot, ionized skin of an accretion disc, suppressing emission of the line. The ionized skin appears as a result of the thermal instability of X-ray irradiated plasma. We test this hypothesis by computing the Thomson thickness of the hot skin on top of the 'alpha P_tot' Shakura-Sunyaev disc, by simultaneously solving the vertical structure of both the hot skin and the disc. We then compute a number of relations between observable quantities, e.g. the hard X-ray flux, amplitude of the observed reprocessed component, relativistic smearing of the K alpha line, the r.m.s. variability of the hard X-rays. These relations can be compared to present and future observations. We point out that this mechanism is unlikely to explain the behaviour of the X-ray source in MCG-6-30-15, where there is a number of arguments against the existence of a thick hot skin, but it can work for some other Seyfert 1 galaxies.Comment: 12 pages. MNRAS, accepte