  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.
