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J. A. Tomsick (Center for Astrophysics and Space Sciences, Univ. of California, San Diego), P. Kaaret (Harvard-Smithsonian Center for Astrophysics)
We present a study of the properties of strong 0.8-3.0 Hz quasi-periodic oscillations (QPOs) that occurred during 1997 Rossi X-ray Timing Explorer observations of the microquasar and black hole candidate GRS 1915+105 in the low-hard state. The high PCA count rates allow us to track individual QPO peaks, and we have exploited this to develop a QPO folding technique. Our technique allows us to measure QPO amplitudes, phase lags and, for the first time, the phase of the QPO harmonics relative to the fundamental. In contrast to previous QPO studies with RXTE, we emphasize the high energy QPO properties and report the detection of a QPO in the 60-124 keV energy band. The strength of the QPO fundamental increases up to about 20 keV, but remains constant or decreases above 20 keV. Possible implications for accretion disk geometries and coronal temperature profiles will be discussed. Also, comparisons to another black hole candidate, XTE J1550-564, will be presented.
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