Spectral Variability in Seyferts from Variable UV Irradiation

Andrzej A. Zdziarski

N. Copernicus Astronomical Center, Warsaw, Poland


A common pattern of variability in Seyferts in the $\sim 1$--10 keV range 
consists of softening of the continuum with an increase of the flux. We model 
it quantitatively with variable UV irradiation of  a cloud of hot thermal 
electrons (presumably an inner hot disk) with a constant Thomson optical depth. 
The electron temperature is determined self-consistently from the balance of 
constant heating and cooling by thermal Comptonization of the UV photons. This 
model has been successfully applied to the variability of 3C 120 observed by 
{\it BeppoSAX}, in which case the spectral index increased from $\sim 1.7$ to 
$\sim 2$ with the increasing soft X-ray flux. The likely source of the UV 
photons is an outer cold disk with a variable truncation radius. An associated 
effect is an increase of the strength of Compton reflection (from the disk) 
with the softening of the spectrum.