High-Frequency Quasi-Periodic Oscillations in the 2000 Outburst of the Galactic Microquasar XTE J1550-564

We present an analysis of the high-frequency timing properties of the April-May 2000 outburst of the black hole candidate and Galactic microquasar XTE J1550-564, measured with the Rossi X-ray Timing Explorer, The rapid X-ray variability we measure is consistent with the source being in either the "very high" or "intermediate" canonical black hole state. A strong (5-8% RMS) quasi-periodic oscillation (QPO) is found between 249-278 Hz; this may represent the first recurrence of the same high-frequency QPO in subsequent outbursts of a transient black hole candidate. We also present possible evidence for a lower-frequency QPO at approximately 187 Hz, also reported previously and likely present simultaneously with the higher-frequency QPO. We discuss these findings within the context of the 1998 outburst of XTE J1550-564, and comment on implications for models of QPOs, accretion flows, and black hole spin.