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D. M. Smith, D. S. Main (UCB/SSL), W. A. Heindl (UCSD/CASS), J. H. Swank (NASA/GSFC)
We have monitored the three known Galactic black-hole candidates which are usually bright and usually in the hard state: 1E1740.7-2942, GRS1758-258, and Cyg X-1. These data from the Rossi X-Ray Timing Explorer have revealed a type of transition common to all three sources: episodes of modest brightening (~40%) and modest softening (photon power law index steepens from ~1.5 to ~1.8) in the range 2.5-25 keV. In all three sources the brightening lasts on the order of a month, but in the two Galactic-bulge jet sources the softening is delayed by about a month from the beginning of the brightening, while in Cyg X-1 the delay is only about 5 hr. We interpret these behaviors in terms of simultaneous accretion via a thin disk and an advection-dominated accretion flow (ADAF). When mass transfer from the companion increases, the ADAF (which provides the hardest emission) brightens immediately. The enhancement in the thin disk propagates at the viscous timescale, and reaches the inner regions a month later, providing the softening. Cyg X-1, as a wind accretor, is expected to have a much smaller thin disk, which fits nicely with the much shorter delay measured for that source.
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