Multifrequency Observations of the Galactic Microquasars GRS1915+105 and GROJ1655-40

Abstract

The two galactic `microquasars' with superluminal radio jets have been quite active during 1996, generatinga variety of studies involving both NASA and ground-based observatories. GRS~1915+105 has displayed dramatic accretion instability in observations with RXTE, revealing X-ray light curves and emission states unlike anything previously seen. Variable QPOs in the range of 0.07--10 Hz have been monitored with the capability to track the individual oscillations. The QPO amplitude is as high as 40% of the mean flux, while both amplitude and phase lag increase with photon energy. The results imply a direct link between the QPO mechanism and the origin of the energetic electrons believed to radiate the X-ray power-law component. GRS1915+105 also displays a transient yet stationary QPO at 67 Hz. The other source, GRO J1655-40, is an optically established black hole binary. Recent optical reports include an excellent model for the binary inclination and masses, while an optical precursor to the April 1996 X-ray outburst has been measured. We report new results from recent RXTE observations. GROJ1655-40 has displayed both the canonical ``soft/high'' and ``very high'' X-ray states, with QPOs at 8--22 Hz during the latter state. In addition, there is a high-frequency QPO at 300 Hz. The rapid oscillations in these sources are suspected of providing a measure of the mass and rotation of the accreting black holes, although several competing models may be applied when evaluating the results.