X-ray variability of AGN and correlations with spectral properties
  Rapid flux changes in the X-ray emission from active galactic nuclei are commonly observed. The power-density spectra show a pseudo power-law form with a turnover at low frequencies and a high frequency break, similar to galactic black-hole candidates. There have been a few claims for periodicities but these are not a well-established property of the class. The amplitude of variability depends on a number of source parameters, including luminosity and spectral properties. The variability amplitude is correlated with X-ray spectral index, and anti-correlated with the width of the permitted optical lines. In one particularly well-observed case, NGC 7469, we see a relationship between the X-ray and UV variability, which indicates that the dominant emission process for the X-rays is thermal Comptonization. Variations in the X-ray emission are related to changes in the UV seed photons, but it appears there must also be a mechanism - perhaps that which heats the Comptonizing corona - that induces rapid variability intrinsic to the X-rays. 
K. Nandra