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R. A. Remillard, E. H. Morgan (M.I.T.)
X-ray timing and spectral analyses of RXTE observations are reported for three X-ray outbursts from the recurrent X-ray nova 4U1630-47. The first (1996) and third (1999) outbursts yield X-ray spectra that are dominated by thermal emission attributed to the accretion disk. However, during the second outburst (1998) we find increased strength and variability in the hard X-ray power-law. We also detect quasi-periodic oscillations (QPO) in the range of 1-10 Hz, as reported previously by Dieters et al.(1998). Furthermore, during a portion of the 1998 outburst, the source exhibits another QPO at 184 Hz. This is the fourth black-hole candidate known to exhibit oscillations faster than 60 Hz. The rms amplitude (1.5 ± 0.3 % at 6-20 keV) and the value of the coherence parameter (frequency / FWHM = 3.7) resemble oscillations seen in GRO J1655-40 and XTE J1550-564. We summarize the status of ongoing efforts to explain these oscillations as a consequence of effects of General Relativity in the inner accretion disk. We also discuss implications for the divergent tracks in spectral evolution exhibited in successive outbursts.
The author(s) of this abstract have provided an email address for comments about the abstract: rr@space.mit.edu