XTE J1550-564

A quasiperiodic oscillation (QPO) in the flux of this transient x-ray source is seen in power spectra of the BATSE 20-50-keV discriminator rates. On Sept. 9.0 UT, the QPO was centered on 271 +/- 2 mHz, and it had a FWHM of 30 +/- 5 mHz and a fractional rms of 19.2 +/- 2.5 percent. The QPO was first detected on Sept. 7.45 near 55 mHz and has been steadily increasing in frequency, reaching 350 mHz by Sept. 9.3. The QPO appears to have reached the Nyquist frequency (488 mHz) of our discriminator rates before the end of Sept. 9, and is no longer detectable in them.