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 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 timescale tau. This parameter was found to be 0.19s. 
K. Pottschmidt, M. Koenig, J. Wilms, R. Staubert (Institut fuer   Astronomie und Astrophysik, Astronomie, Tuebingen)