X-ray variability of AGN and the flare model

Short-term variability of X-ray continuum spectra has been reported for several Active Galactic Nuclei. Significant X-ray flux variations are observed within time scales down to 10(3) - 10(5) seconds. We discuss short variability time scales in the frame of the X-ray flare model, which assumes the release of a large hard X-ray flux above a small portion of the accretion disk. The resulting observed X-ray spectrum is composed of the primary radiation and of a reprocessed Compton reflection component that we model with numerical radiative transfer simulations. The incident hard X-rays of the flare will heat up the atmosphere of the accretion disk and hence induce thermal expansion. Eventually, the flare source will be surrounded by an optically-thick medium, which should modify the observed spectra.