Correlation between variability time scale and X-ray spectral index in AGN

. We have analysed EXOSAT ME X-ray light curves of a sample of AGN assuming a stochastic process in these sources. The result is a significant anticorrelation between the relaxation timescale ø , derived from this stochastic model fit, and the photon index \Gamma of the X-ray spectrum. This establishes for the first time a relationship between variability properties and spectral properties of the emitted X-ray flux of AGN. Even though the physical mechanism for the variations is far from clear, models employing the stochastic superposition of discrete events appear most promising. We propose Comptonization as a possible cause for the relationship. Key words: X-rays: galaxies 1. Introduction Rapid X-ray variability of active galactic nuclei (AGN) has been considered a valuable source of information (Wallinder, Kato & Abramowicz 1992) about the physics at work in the vicinity of the supermassive black holes (BH) believed to exist in their centers (Rees 1984). The traditional method in ..