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Astron. Astrophys. 334, 201-209 (1998)

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Analyzing short-term X-ray variability of Cygnus X-1
with Linear State Space Models

Katja Pottschmidt , Michael König , Jörn Wilms and Rüdiger Staubert

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

Received 7 October 1997 / Accepted 13 February 1998

Abstract

Cyg X-1 exhibits irregular X-ray variability on all measured timescales. The usually applied shot noise models describe the typical short-term behavior of this source by superposition of randomly occuring shots with a distribution of shot durations. We have reanalyzed EXOSAT ME observations of Cyg X-1 using the more general Linear State Space Models (LSSMs). These models, which explicitly take the observation-noise into account, model the intrinsic system variability with an autoregressive (AR) process. Our fits show that an AR process of first order can reproduce the system variability of Cyg X-1. A possible interpretation is again the superposition of individual shots, but with a single relaxation time [FORMULA]. This parameter was found to be 0.19 s.

Key words: X-rays: stars – binaries: close – accretion, accretion disks – stars: individual: Cyg X-1 – methods: statistical

Send offprint requests to: K. Pottschmidt, (katja@astro.uni-tuebingen.de)

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© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998

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