Temporal properties of Cygnus X-1

We compare observational results from two RXTE observations of the black ho le candidate Cygnus X-l performed in 1996 October and 1997 December. The ob servations have the same intrinsic Bur, but the second observation was made during a considerably harder spectral state and shows evidence for reproce ssing of radiation in cold material. We present results from Fourier timing techniques (power spectral densities and time lags). See Nowak et al. (1999a) for a self-contained introduction to these methods. We verify the Belloni & Hasinger (1990) effect of a vari able break frequency in the PSD. The cross energy analysis shows that the s hape of the time lag can differ substantially between hard state observatio ns of the same intrinsic luminosity.