2003ApJ...592.1100T

We report on X-ray observations made with the Rossi X-Ray Timing Explorer 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 six bright ∼100 s X-ray flares and long timescale oscillations of the X-ray flux. The oscillations are aperiodic with a characteristic timescale of 14.2 days and an order of magnitude variation in the 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. The X-ray flares have durations between 62 and 215 s and peak fluxes that are 5-24 times higher than the persistent flux. The flares have nonthermal energy spectra and occur when the persistent luminosity is near 3x10³⁴(d/4kpc)² ergs/s (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 the X-ray flux increases by a factor of up to 24 on a timescale of seconds. We discuss the flares in the context of observations and theory of Galactic BHCs and compare the flares to those detected from Sgr A*, the supermassive black hole at the Galactic center. 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 neutron star bursts, the flares have nonthermal energy spectra in contrast to the blackbody spectra exhibited in bursts. This indicates that X-ray bursts should not be taken as evidence that a given system contains a neutron star unless the presence of a blackbody component in the burst spectrum can be demonstrated.