ar X iv : a st ro - p h / 02 02 30 5 v 1 1 5 Fe b 20 02 Evidence for Harmonic Relationships in the High Frequency

We continue to investigate the X-ray timing and spectral properties of the black hole binary XTE J1550–564 during its 1998-1999 outburst. By grouping observations according to the type of low-frequency quasiperiodic oscillation (LFQPO) identified in a previous paper, we show evidence that two high-frequency QPOs (HFQPOs) occur simultaneously near 184 and 276 Hz. In each case, we can model the QPO profiles while assuming that the central frequencies are related by a 3:2 ratio. In one group, there is also evidence of a broad feature at the fundamental frequency of 92 Hz. We also investigate the April 2000 outburst of the source and confirm the suggestion of Miller et al. that a 270 Hz QPO is accompanied by a second feature near 180 Hz. We conclude that the majority of the 28 individual HFQPO detections in XTE J1550–564 are best interpreted as harmonics of a fundamental oscillation near 92 Hz. The most significant exceptions to this pattern are the 143 Hz QPO detected on 1998 October 15 and the 65 Hz QPO reported by Kalemci et al. for 2000 May 20. We next investigate all of the X-ray energy spectra and QPO results for individual observations of XTE J1550–564, and we find a systematic increase in the strength of the power-law component as the stronger of the two HFQPOs shifts from 276 to 184 Hz. A strikingly similar result is