Monitoring the Galactic microquasars with RXTE

Abstract

The Galactic microquasars GRS1915+105 and GRO J1655-40 were targets for dedicated monitoring programs with RXT E throughout 1996 and 1997. We review recent results with particular attention to the X-ray QPOs seen in these sources. We confirm the reappearance of 67 Hz oscillations in GRS1915+105 in several different spectral states. Six intervals, each of 1 to 6 months duration, yield QPO detections with a central frequency of 66.7 ± 1.4 Hz and a FWHM of 3.4 ± 1.0 Hz. Comparing these observing intervals, the average X-ray luminosity varies by a factor of 4 with no significant change in the central QPO frequency. GRS1915+105 has been active in X-rays since 1992, and it shows no signs of evolving toward quiescence. In contrast, GRO J1655-40, which began its most recent X-ray outburst on 1996 April 25, is seen to fade below 20 mCrab on 1997 Aug 17. There are no reappearances of the 300 Hz QPO during the entire second half of the X-ray outburst; during this time the X-ray spectrum is dominated by the soft, thermal component. There are still several models seeking to explain the high frequency QPOs in microquasars. While it is likely that the cause invokes some effect of General Relativity, the correct model and the details of the emission mechanism remain uncertain.

Evidence for Intrinsic Magnetic Moments in Black Hole Candidates

“…APPENDIX mass. A 67 Hz QPO has been observed (Remillard et al 1997, Remillard & Morgan 1998 that is sharp (Q > 20) and stable for factors of 5 luminosity change over six months time. If this is the spin frequency, a co-rotation radius of 174 km is implied.…”

Millisecond Oscillations in X-Ray Binaries

“…The 67 Hz QPO in GRS 1915+105 , Remillard & Morgan 1998) varies by only a few percent in frequency when the X-ray flux varies by a factor of several. It is relatively coherent, with Q usually around 20 (but sometimes dropping to 6; Remillard et al 1999a); it has an rms amplitude of about 1%, and the signal at high photon energy lags that at lower energy by up to 2.3 radians (Cui 1999).…”