Studies of the Spectral and Timing Evolution of XTE J1550-564

Recently we reported on timing and spectral observations, including a low-frequency quasi-periodic oscillation (LFQPO), of the 2000 outburst of XTE J1550-564 made by the Unconventional Stellar Aspect (USA) Experiment (See ApJL, 561, L183). The LFQPO of the 2000 outburst has also shown an anti-correlation in rms amplitude with X-rays between 1-4 keV and a correlation with high energy X-rays from BATSE. RXTE data, from the 1998-1999 outburst, shows similar behavior. These observations could be consistent with a LFQPO which is being produced, as magneto-acoustic oscillations, in a comptonizing region surrounding the central object. If so, then the increase in LFQPO frequency may indicate that the comptonizing region is shrinking, due to the inner edge of the accretion disk moving closer to the central object, as the source reaches its maximum luminousity. The anti-correlation of LFQPO rms amplitude is also consistent with the typical outburst scenario. As the disk becomes more luminous, soft photons cool the inner comptonizing region and inhibit the source of the LFQPO photons. Based on these observations we hope to measure the radius of the inner edge of the accretion disk.