Evidence for Harmonic Relationships in the High Frequency QPOs of XTE J1550-564 and GRO J1655-40

We continue to investigate the X-ray timing and spectral properties of the black hole binary, XTE J1550-564. For three different groups of observations, we show evidence that two high-frequency QPOs (HFQPOs) occur simultaneously near 184 and 276 Hz. In one group, there is also evidence of a broad feature at 92 Hz. In each case, we can model the QPO profiles while assuming that the central frequencies are related as integral harmonics of a single frequency. We next investigate the X-ray energy spectra, 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 seen for GRO J1655-40 when the stronger HFQPO shifts from 450 to 300 Hz. The fundamental HFQPO frequencies for the two X-ray sources scale inversely with black hole mass, which is consistent with the hypotheses that these HFQPOs represent some oscillation rooted in general relativity (GR) and that the two black holes have similar values of the dimensionless spin parameter. We discuss physical mechanisms that may explain these HFQPOs. In particular, a resonance between the orbital and radial coordinate frequencies in the innner accretion disk, as proposed by Abramowicz & Kluzniak, would imply moderate values for the dimensionless spin parameter (0.1 < a < 0.6) for both black holes.