X-ray timing and spectral observations of galactic black hole candidate XTE J1550-564 during outburst

The timing and spectral properties of the soft X-ray transient (SXT) and microquasar XTE J1550-564 during outburst are studied, emphasizing observations made by the Unconventional Stellar Aspect (USA) Experiment. USA data show a low-frequency quasi-periodic oscillation (LFQPO) with a centroid frequency that tends to increase with increasing flux and a fractional rms amplitude which is correlated with the USA hardness ratio. Several high- frequency quasi-periodic oscillations (HFQPOs) were detected by RXTE, during periods where the LFQPO is seen to be weakening or not detectable at all. The evolution of the USA hardness ratio with time and source flux is examined. The hardness-intensity diagram shows counterclockwise cyclical evolution and possibly indicates the presence of two independent accretion flows: a geometrically thin, optically thick accretion disk and a hot sub-Keplerian flow. A model for production of the LFQPO in XTE J1550-564 is proposed, assuming that the LFQPO is a result of acoustic oscillations in a hot sub-Keplerian transition layer (TL). Under this acoustic oscillation model (AOM), the TL is an acoustic cavity having an outer boundary at R_TL, where R _TL ≈ R_inner, with R_inner being the radius of the accretion disk inner edge. TL resonance is interpreted as a mechanism for the LFQPO and the eigenfrequencies are associated with the LFQPO frequencies, ν _LFQPO. This LFQPO mechanism requires the presence of a TL, capable of inverse-Compton scattering, with RTL regulated by a cool Keplerian accretion disk. Support for the AOM is provided by the correlation of the LFQPO percent rms amplitude with the USA hardness ratio, and other correlations and anti-correlations found in the USA, RXTE and BATSE observations. LFQPO data show increasing rms amplitude with increasing high energy flux and increasing ν_LFQPO with increasing low energy flux. These observations indicate that ν _LFQPO is set by the accretion disk inner edge. The spectrally inferred R_inner is commensurate, showing a decrease with increasing ν _LFQPO. Finally, RXTE spectral data indicate that the LFQPO is detectable only when the power law contribution is greater than 20% of the low energy flux, providing evidence that the LFQPO mechanism is dependent on the TL. The AOM is consistent with all of the above observations and it has the ability to predict the approximate value of R_inner. Comparison between spectrally determined R_inner and the AOM measurement of RTL shows good agreement.