X-Ray Observations of XTE J1550–564 during the Decay of the 2000 Outburst. II. Timing

We investigate the timing behavior of XTE J1550-564 with the Rossi X-Ray Timing Explorer as the source made a transition to the hard state during the decay of the 2000 outburst. We detect a high-frequency quasi-periodic oscillation (QPO) at 65 Hz in one observation with a fractional rms amplitude of 4.9%. This is the first time that a high-frequency QPO has been detected in a transition to the hard state during outburst decay. We also observe low-frequency QPOs in the 0.36-4.1 Hz range and rich aperiodic variability. The changes in the temporal properties during the decay are very similar to the state transitions observed for other sources. We find a correlation between the low-frequency QPO and the break frequency in the continuum power spectra. We investigate the energy dependence of rms amplitudes of the QPOs. We compare these timing properties with those previously observed for XTE J1550-564 and other Galactic black hole candidates and discuss the implications for QPO models and black hole accretion.