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Astron. Astrophys. 326, L25-L28 (1997)

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Letter to the Editor

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

M. König , R. Staubert and J. Wilms

Institut für Astronomie und Astrophysik, Astronomie, University of Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

Received 26 June 1997 / Accepted 8 July 1997


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 anti-correlation between the relaxation timescale [FORMULA], derived from this stochastic model fit, and the photon index [FORMULA] 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

Send offprint requests to: M. König

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998