Properties of a fast state transition in Cygnus X‐1

In 2005 February we observed the microquasar Cygnus X‐1 for a total of 10 days quasi‐continuously with the Rossi X‐ray Timing Explorer and the Ryle telescope. During that period of time Cyg X‐1 was very variable and covered a large fraction of its total variability found during our years long monitoring. A full transition from the hard to the soft state occured within less than 2.5 hours. We present results of the spectral and timing analysis, compare them with the long‐term behavior and discuss the discovered energy dependence of the power spectra.