  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.
