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D. A. Smith, R. Remillard (MIT)
We report on X-ray observations of V4641 Sgr carried out in September 1999 with the instruments of the Rossi X-ray Timing Explorer. Following the optical brightening that began on 1999 Sept. 15.4, the X-ray intensity was found by the ASM (2--12 keV) to rise within 7 hr to a flux of 12 Crab on Sept. 15.7, and then to vanish within two hours. Rapidly scheduled PCA / HEXTE observations (Sept. 15.9) revealed another 20 min outburst that resembles a succession of dramatic, fast flares with variability on timescales shorter than one second. Before the end of these pointed observations, the X-ray source decreases to the PCA detection threshold. The average X-ray spectrum during the later outburst can be successfully modeled with a typical accreting black-hole spectrum consisting of a multi-temperature disk blackbody, an iron emission line, and a cut-off power law. However, the power law component is much flatter than usual, with a photon index of 0.8 and a cutoff energy around 45 keV. During the rise of the outburst, the peak in the spectral energy distribution shifts from 35 keV to about 55 keV. Subsequent monitoring observations by Hjellming et al. (1999) have shown the emergence of bipolar radio jets. V4641 Sgr may represent a possible new subclass of X-ray binaries that show jets combined with X-ray flares that decay at time scales 30 to 100 times faster than typical X-ray novae. The 4-year light curve from the ASM shows evidence of earlier flares, although none are as dramatic as those that occurred during 1999 Sept. Efforts to reconcile the spectral model with the extremely fast temporal variations are discussed.
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The author(s) of this abstract have provided an email address for comments about the abstract: dasmith@space.mit.edu