TEMPORAL PROPERTIES OF CYGNUS X-1 DURING THE SPECTRAL TRANSITIONS

We report the results from our timing analysis of 15 Rossi X-Ray Timin g Explorer observations of Cygnus X-1 throughout its 1996 spectral tra nsitions. The entire period can be divided into three distinct phases: (1) transition from the hard state to the soft state, (2) soft state, and (3) transition from the soft state back to the hard state. The ob served X-ray properties (both temporal and spectral) in Phases 1 and 3 are remarkably similar, suggesting that the same physical processes a re likely involved in triggering such transitions. The power density s pectrum (PDS) during the transition can be characterized by a low-freq uency red-noise (power-law) component, followed by a white-noise (flat ) component that extends to roughly 1-3 Hz, where it is cut off, and a steeper power law (similar to 1/f(2)) at higher frequencies. The X-ra y flux also exhibits apparent quasi-periodic oscillations (QPOs), with the centroid frequency varying in the range of 4-12 Hz. The QPO shows no correlation with the source flux, but it becomes more prominent at higher energies. This type of PDS bears resemblance to that of other black hole candidates often observed in a so-called very high state, a lthough the origin of the observed QPO may be very different. The low- frequency red noise has not been observed in the hard state, and thus seems to be correlated positively with the disk mass accretion rate, w hich is presumably low in the hard state and high in the soft state; i n fact, it completely dominates the PDS in the soft state. In the fram ework of thermal Comptonization models, Cui et al. recently speculated that the difference in the observed spectral and timing properties be tween the hard and soft states is due to the presence of a ''fluctuati ng'' Comptonizing corona during the transition. Here we present the me asured hard X-ray time lags and coherence functions between various en ergy bands, and we show that the results strongly support such a scena rio.