Temporal Evolution of X-ray Lags in Cygnus X-1
  We present the long term evolution of the frequency-dependent X-ray time lags of the black hole candidate Cygnus X-1 as measured in 1996 and 1998 with the Rossi X-ray Timing Explorer (RXTE). Lag spectra measured during the 1996 June soft state are very similar to those seen during 1996 December and most 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 tends 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 the state transitions might be related to the formation and destruction of the synchrotron radiation emitting outflows present during the hard state. 
K. Pottschmidt (1), J. Wilms (1), M. A. Nowak (2), W. A. Heindl (3),   D. M. Smith (4), R. Staubert (1) ((1) IAA Tuebingen, ...