Discovery of Submillisecond Quasi-periodic Oscillations in the X-Ray Flux of Scorpius X-1

We report the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), ofthe first sub-millisecond oscillation found in a celestial X-ray source. Thequasi-periodic oscillations (QPO) come from Sco X-1 and have a frequency ofapproximately 1100 Hz, amplitudes of 0.6-1.2% (rms) and are relativelycoherent, with Q up to ~10^2. The frequency of the QPO increases with accretionrate, rising from 1050 to 1130 Hz when the source moves from top to bottomalong the normal branch in the X-ray color-color diagram, and shows a strong,approximately linear correlation with the frequency of the well-known 6-20 Hznormal/flaring branch QPO. We also report the discovery of QPO with a frequencynear 800 Hz that occurs, simultaneously with the 1100 Hz QPO, in the uppernormal branch. We discuss several possible interpretations, one involving amillisecond X-ray pulsar whose pulses we see reflected off accretion-flowinhomogeneities. Finally, we report the discovery of ~45 Hz QPO, most prominentin the middle of the normal branch, which might be magnetosphericbeat-frequency QPO.