2001ApJ...563..928M

We present an analysis of the high-frequency timing properties of the 2000 April-May 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'' black hole state. Strong (5%-8% rms) quasi-periodic oscillations (QPOs) are found between 249 and 276 Hz; this represents the first detection of the same high-frequency QPO in subsequent outbursts of a transient black hole candidate. We also present evidence for lower frequency QPOs at approximately 188 Hz (3.5 σ, single trial), also reported previously and likely present simultaneously with the higher frequency QPOs. 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.