Institut für Astronomie und Astrophysik

Abteilung Astronomie

Preprint 13/97

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, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

accepted by Astronomy & Astrophysics

Abstract. 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 tau, 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

Paper (124k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de) Last updated 06-Oct-1997