USA Experiment Observation of Spectral and Timing Evolution during the 2000 Outburst of XTE J1550−564

We report on timing and spectral observations of the 2000 outburst of XTE J1550-564 made by the Unconventional Stellar Aspect (USA) experiment on board the Advanced Research and Global Observation Satellite ( ARGOS ). We observe a low-frequency quasi-periodic oscillation with a centroid frequency that tends to increase with increasing flux and a fractional rms amplitude that is correlated with the hardness ratio. Several high-frequency quasi-periodic oscillations were detected by Rossi X-Ray Timing Explorer during periods where the low-frequency quasi-periodic oscillation was seen to be weakening or not detectable at all. The evolution of the hardness ratio (4-16 keV/1-4 keV) with time and source flux is examined. The hardness-intensity diagram shows a cyclical movement in the counterclockwise direction and possibly indicates the presence of two independent accretion flows: a thin-disk flow and a hot sub-Keplerian flow.