  We have investigated the X-ray timing properties of XTE J1550-564 during 60
RXTE PCA observations made between 1998 September 18 and November 28. We detect
quasi-periodic oscillations (QPOs) near 185 Hz during four time intervals. The
QPO widths (FWHM) are near 50 Hz, and the rms amplitudes are about 1% of the
mean flux at 2-30 keV. This is the third Galactic black hole candidate known to
exhibit a transient X-ray timing signature above 50 Hz, following the 67 Hz QPO
in GRS1915+105 and the 300 Hz QPO in GRO J1655-40. However, unlike the previous
cases, which appear to show stationary frequencies, the QPO frequency in XTE
J1550-564 must vary by at least 10% to be consistent with observations. The
occurrences and properties of the QPO were insensitive to large changes in the
X-ray intensity (1.5 to 6.8 Crab). However, the QPO appearance was accompanied
by changes in the energy spectrum, namely, an increase in the temperature and a
decrease in the normalization of the thermal component. The QPO is also closely
related to the hard X-ray power-law component of the energy spectrum since the
fractional amplitude of the QPO increases with photon energy. The fast QPOs in
accreting black hole binaries are thought to be effects of general relativity;
however, the relevance of the specific physical models that have been proposed
remains largely uncertain. Low frequency QPOs in the range 3-13 Hz were often
observed. Occasionally at high luminosity the rms QPO amplitude was 15% of the
flux, a level previously reached only by GRS1915+105.
