ANALYZING SHORT-TERM X-RAY VARIABILITY OF CYGNUS X-1 WITH LINEAR STATE-SPACE MODELS

Cyg X-1 exhibits irregular X-ray variability on all measured timescale s. The usually applied shot noise models describe the typical short-te rm behavior of this source by superposition of randomly occuring shots with a distribution of shot durations. We have reanalyzed EXOSAT ME o bservations of Cyg X-1 using the more general Linear State Space Model s (LSSMs). These models, which explicitly take the observation noise i nto account, model the intrinsic system variability with an autoregres sive (AR) process. Our fits show that an AR process of first order can reproduce the system variability of Cyg X-1. A possible interpretatio n is again the superposition of individual shots, but with a single re laxation time tau. This parameter was found to be 0.19 s.