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J.A. Tomsick (CASS/UCSD), E. Kalemci (SSL/UCB), S. Corbel (Universite Paris VII and CEA Saclay), P. Kaaret (Harvard-Smithsonian CfA)
We report on X-ray observations made with RXTE and Chandra of the black hole candidate (BHC) transient XTE J1650-500 at the end of its first, and currently only, outburst. By monitoring the source at low luminosities over several months, we found new or previously poorly studied BHC X-ray phenomena, including bright ~100 s X-ray flares and long time scale oscillations of the X-ray flux. The oscillations are aperiodic with a characteristic time scale of 14.2 days and a peak-to-peak amplitude of approximately an order of magnitude in 2.8-20 keV flux. The oscillations may be related to optical "mini-outbursts" that have been observed at the ends of outbursts for other short orbital period BHC transients. Also, we found 6 X-ray flares with durations between 62 and 215 s and peak fluxes that are 5-24 times higher than the persistent flux. The flares have non-thermal energy spectra and occur when the persistent luminosity is near 3E34 (d/4 kpc)2 erg s-1 (2.8-20 keV). The rise time for the brightest flare demonstrates that physical models for BHC systems must be able to account for the situation where up to 96% of the X-ray flux from the source changes on a time scale of seconds. In addition to discussing the flares in the context of Galactic BHCs, we compare the flares to those that have been detected from Sgr A*. We also compare the flares to X-ray bursts that are seen in neutron star systems. While some of the flare light curves are similar to those of bursts, the non-thermal energy spectra distinguish them from the blackbody spectra exhibited by bursts.
Support for this work was provided by NASA through grant NAG5-10886 and Chandra award number GO2-3056X issued by the Chandra X-ray Observatory Center, which is operated by SAO for and on behalf of NASA under contract NAS8-39073.
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Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.