[Previous] | [Session 108] | [Next]
D. M. Smith (U. C. Berkeley), W. A. Heindl (U. C. San Diego), J. H. Swank (NASA/GSFC)
The Galactic-bulge x-ray sources 1E 1740.7-2942 and GRS 1758-258 are the only black-hole candidates besides Cyg X-1 to display long-term persistent emission with a hard spectrum. Unlike Cyg X-1, they have prominent radio jets, and have thus been called "microquasars". Nothing is known about the companion of either source due to the high extinction and great distance to the bulge. Now we have the first x-ray evidence which may allow us to guess about their companions: a weak periodic modulation has been detected in long-term x-ray lightcurves of both sources. The data are from the Proportional Counter Array on the Rossi X-ray Timing Explorer. The periods are roughly 12.5 dy in 1E 1740.7-2942 and 18.5 dy in GRS 1758-258. We will discuss our observations and possible intepretations of the periodicities as being orbital periods, disk-precession periods, or the periods of a superhump phenomenon.