X-ray Variability of Active Galactic Nuclei and the Flare Model
  Short-term variability of X-ray spectra has been reported for several Active Galactic Nuclei. Significant X-ray flux variations are observed within time scales down to 10^4 -- 10^5 seconds. We discuss short variability time scales in the framework 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 as well as a reprocessed/Compton reflection component that we model performing numerical radiative transfer simulations. We conduct Monte-Carlo simulations of large flare distributions across the disk including relativistic corrections of the observed radiation and discuss general implications on AGN spectra and their variability. 
R.W. Goosmann, B. Czerny, A.-M. Dumont, M. Mouchet, A. Rozanska, V.   Karas, M. Dovciak