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Wei Cui (MIT), S. N. Zhang (University of Alabama-Huntsville), Wan Chen (GSFC/NASA and University of Maryland), J. H. Swank (GSFC/NASA)
The study of quasi-periodic oscillations (QPOs) in black hole candidates (BHCs) is one of the most important topics that RXTE has contributed significantly. Prior to the launch of RXTE, the QPOs were observed only at very low frequencies (< 1 Hz), except for the ``very-high-state'' QPOs (at a few Hz) observed only on two occasions. RXTE has pushed the discovery space up to hundreds of Hz. New QPOs have been discovered initially in microquasars, GRS 1915+105 and GRO J1655-40, and then in ``normal'' BHCs, Cyg X-1, GRS 1758-258, 1E 1740.7-2942, 4U 1630-47, XTE J1748-2848, and XTE J1550-564. We will present the most recent results on several sources, based on data from a dedicated RXTE program we have set up for systematically studying soft X-ray transients during the rise of an outburst. We will show that the data have begun to allow quantitative characterization of the QPOs and, thus, to demand quantitative models to explain the observed properties. From the derived energy dependence of the QPO amplitude and the correlation between QPO properties and mass accretion rate, it is clear that the QPOs in BHCs consist of a heterogeneous class of phenomena, although the origins are hardly understood. Several proposed models will be examined.