X‐Ray Flares and Oscillations from the Black Hole Candidate X‐Ray Transient XTE J1650−500 at Low Luminosity

We report on X-ray observations made with the Rossi X-ray Timing Explorer ofthe 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 overseveral months, we found 6 bright ~100 second X-ray flares and long time scaleoscillations of the X-ray flux. The oscillations are aperiodic with acharacteristic time scale of 14.2 days and an order of magnitude variation inthe 2.8-20 keV flux. The oscillations may be related to optical"mini-outbursts" that have been observed at the ends of outbursts for othershort orbital period BHC transients. The X-ray flares have durations between 62and 215 seconds and peak fluxes that are 5-24 times higher than the persistentflux. The flares have non-thermal energy spectra and occur when the persistentluminosity is near 3E34 (d/4 kpc)^2 erg/s (2.8-20 keV). The rise time for thebrightest flare demonstrates that physical models for BHC systems must be ableto account for the situation where the X-ray flux increases by a factor of upto 24 on a time scale of seconds. We discuss the flares in the context ofobservations and theory of Galactic BHCs and compare the flares to thosedetected from Sgr A*, the super-massive black hole at the Galactic center. Wealso 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 starbursts, the flares have non-thermal energy spectra in contrast to the blackbodyspectra exhibited in bursts. This indicates that X-ray bursts should not betaken as evidence that a given system contains a neutron star unless thepresence of a blackbody component in the burst spectrum can be demonstrated.Comment: 9 pages, accepted by Ap