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K. S. Wood (E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375), USA Experiment Science Working Group
Several features make the USA Experiment on ARGOS well-suited to observation of transient black hole candidates. Its pointing schedule can be revised on a few days notice to permit extensive observations of new transients, extending over many weeks and with coverage up to once or twice per orbit as needed. The sampling frequency is particularly useful when the power spectral density is evolving dynamically on timescales of hours to days. USA also provides spectral coverage below 3 keV for isolation of disk components of sources. To date, USA has been used to observe XTE J1859+226, XTE J1550-564 and XTE J1118+480 as well as more persistent black hole candidates such as X1630-472, GRS1915+105, GX339-4 and Cyg X-1. This talk reviews these observations, merging them with those of RXTE where appropriate. In particular, XTE J1118+480 has been monitored in this manner during its recent outburst beginning April 2000. We observe evolution of a low frequency QPO which drifts from 0.07 Hz to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source and compare this QPO to other low frequency black hole candidates.
Basic research in X-ray Astronomy at the Naval Research Laboratory is supported by NRL/ONR. Work on USA at SLAC is supported by Department of Energy contract DE-AC03-76SF00515.
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