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

\.{Z}ycki P. and R\'{o}\.{z}a\'{n}ska A.

Copernicus Astronomical Center, Warsaw, Poland


A number of recent results from X--ray observations of Active Galactic
Nuclei involving the Fe K$_{\alpha}$ line (reduction of the 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 $\alpha P_{\rm tot}$
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.