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-raytransient 4U 1630-47 for 51 RXTE observations made during the decay of its 1998outburst. The observations began when the source was still relatively bright,and, during one of the early observations, a QPO with a non-Lorentzian profileoccurred near 6 Hz. As the source decayed, the X-ray flux dropped exponentiallywith an e-folding time of 14.4 d. The exponential decay was interrupted by anincrease in the X-ray flux, and a secondary maximum occurred 89 d after theonset of the outburst. A transition marked by significant changes in the timingand spectral properties of the source occurred 104 d after the start of theoutburst. The transition is similar to soft-to-hard state transitions observedin other black hole candidate X-ray binaries. Most of the changes associatedwith the transition occurred in less than 2 d. The timing changes include anincrease 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 anRMS amplitude of 7.3% in the 2-21 keV energy band. At the transition, theenergy spectrum also changed with an abrupt drop in the soft component flux inthe 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. Afterthe transition, the source flux continued to decrease, and the QPO frequencydecayed gradually from 3.4 Hz to about 0.2 Hz.Comment: Accepted for publication in ApJ, 14 pages, 10 figures, uses emulateapj style fil