Temporal evolution of X-ray lags in Cygnus X-1

We present the long term evolution of the frequency-dependent X-ray time la gs of the black hole candidate Cygnus X-1 as measured in 1996 and 1998 with the Rossi Xray Timing Explorer (RXTE). Lag spectra measured during the 199 6 June soft state are very similar to those seen during 1996 December and m ost of 1998 while Cyg X-1 was in its hard state. During state transitions, however, the shape and magnitude of the X-ray lag is highly variable and te nds to be much larger than outside of the state transitions. This behavior is most obvious in the 1-10 Hz band. The increase of the X-ray lag during t he state transitions might be related to the formation and destruction of t he synchrotron radiation emitting outflows present during the hard state.