A Peculiar Flaring Episode of Cygnus X-1

Recent monitoring of Cyg X-1 with {\em RXTE} revealed a period of intenseflaring, which started in October of 2000 and lasted until March of 2001. Thesource exhibited some quite unusual behaviors during this period. The softX-ray flux of the source went up and down three times on a timescale of aboutone month, as discovered by the ASM aboard RXTE, before finally returning tothe normal level (of the hard state). The observed spectral and temporal X-rayproperties of Cyg X-1 are mostly intermediate between the canonical hard andsoft states. This is known previously for strong X-ray flares, however, we showthat the source did enter a period that resembles, in many ways, a sustainedsoft state during the last of the three flares. We make detailed comparisonsbetween this flare and the 1996 state transition, in terms of the observedX-ray properties, such as flux--hardness correlation, X-ray spectrum, and powerdensity spectrum. We point out the similarities and differences, and discusspossible implications of the results on our understanding of the phenomena offlares and state transitions associated with Cyg X-1.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter