X-Ray Spectral and Timing Evolution During the Decay of the 1998   Outburst from the Recurrent X-Ray Transient 4U 1630-47
  We report on the X-ray spectral and timing behavior of the recurrent X-ray transient 4U 1630-47 for 51 RXTE observations made during the decay of its 1998 outburst. The observations began when the source was still relatively bright, and, during one of the early observations, a QPO with a non-Lorentzian profile occurred near 6 Hz. As the source decayed, the X-ray flux dropped exponentially with an e-folding time of 14.4 d. The exponential decay was interrupted by an increase in the X-ray flux, and a secondary maximum occurred 89 d after the onset of the outburst. A transition marked by significant changes in the timing and spectral properties of the source occurred 104 d after the start of the outburst. The transition is similar to soft-to-hard state transitions observed in other black hole candidate X-ray binaries. Most of the changes associated with the transition occurred in less than 2 d. The timing changes include an increase in the continuum noise level from less than 4% RMS to greater than 10% RMS and the appearance of a quasi-periodic oscillation (QPO) at 3.4 Hz with an RMS amplitude of 7.3% in the 2-21 keV energy band. At the transition, the energy spectrum also changed with an abrupt drop in the soft component flux in the RXTE band pass. A change in the power-law photon index from 2.3 to 1.8, also associated with the transition, occurred over a time period of 8 d. After the transition, the source flux continued to decrease, and the QPO frequency decayed gradually from 3.4 Hz to about 0.2 Hz. 
John A. Tomsick (Columbia University, now at CASS/UCSD), Philip Kaaret   (Harvard-Smithsonian CfA)