Cygnus X-1 from RXTE: monitoring the short term variability

Abstract We present temporal and spectral results from monitoring Cygnus X-1 with the Rossi X-ray Timing Explorer (RXTE) in 1998 and 1999. We concentrate on the long term evolution of the hard state timing properties, comparing it to the 1996 soft state evolution. This leads to the following results: 1. the hard and soft state time lag spectra are very similar, 2. during state transitions, the lags in the 1–10 Hz range increase by more than an order of magnitude, 3. in the hard state itself, flaring events can be seen — the temporal and spectral evolution during the flare of 1998 July identifies it as a “failed state transition”. During (failed) state transitions, the time lag spectra and the power spectra change predominantly in the 1–10 Hz range. We suggest that this additional variability is produced in ejected coronal material disrupting the synchrotron radiation emitting outflows present in the hard state.