Toy model of obscurational variability in active galactic nuclei

We propose an explanation of the variability of AGN based on a cloud model of accretion onto a black hole. These clouds, present at a distance of 10-1 00 r(Schw), possibly come from the disrupted innermost disk and they can pa rtially obscure the central X-ray source due to their extreme Compton optic al depth. We consider the implications of this scenario using a toy model o f AGN spectra and support the results with full radiative transfer computat ions from the codes TITAN and NOAR. We show that small random rearrangement s of the cloud distribution can happen on timescales of the order of 10(2) - 10(6) s and may lead to relatively high variability amplitude in the X-ra y band if the mean covering factor is large. The normalized variability amp litude in the UV band is either the same as in the X-ray band, if the contr ibution from the dark sides of the clouds to the UV band is negligible, or it is smaller. The X-ray spectrum should basically preserve its spectral sh ape even during the high amplitude variability, within the frame of our mod el.