New insight into AGN by combining spectral and timing information

A case is made for combining spectral and timing analysis of X-ray data in order to better understand the physics at work in Active Galactic Nuclei (A GN). In analysing X-ray light curves from long EXOSAT ME observations of a sample of AGN with a new technique (assuming a stochastic behaviour of the sources) we determine characteristic variability time constants and find th at there is a significant correlation with the 2-10 keV spectral index (Kon ig et al. 1997). This establishes a relationship between the photon index o f the X-ray spectrum, Gamma, and the fastness of the variability. Similar r elations have been noted between Gamma and the variability amplitude (Green et al. 1993, Fiore et al. 1998).