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 assum ing a stochastic process in these sources. The result is a significant anti-correlation between the relaxation timescale tau, derived from t his stochastic model fit, and the photon index Gamma of the X-ray spec trum. This establishes for the first time a relationship between varia bility properties and spectral properties of the emitted X-ray flux of AGN. Even though the physical mechanism for the variations is far fro m dear, models employing the stochastic superposition of discrete even ts appear most promising. We propose Comptonization as a possible caus e for the relationship.