X-Ray Spectral and Timing Evolution During the Decay of the 1998 Outburst from the Recurrent X-Ray Transient 4U 1630-47

Abstract

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.

Accepted for publication in ApJ, 14 pages, 10 figures, uses emulateapj style file

References in corpus (2)

• Discovery of a New Soft Gamma Repeater, SGR 1627-41
• Precise Interplanetary Network Localization of a New Soft Gamma Repeater, SGR1627-41

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