We present the calibration and background model for the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. The energy calibration is systematics-limited below 10 keV, with deviations from a power-law fit to the Crab Nebula plus pulsar of less than 1%. Unmodeled variations in the instrumental background amount to less than 2% of the observed background below 10 keV and less than 1% between 10 and 20 keV. Individual photon arrival times are accurate to 4.4 s at all times during the mission and to 2.5 s after 1997 April 29. The peak pointing direction of the five collimators is known to a precision of a few arcseconds.

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We present the 2-60 keV spectrum of the supernova remnant Cassiopeia A measured using the Proportional Counter Array and the High Energy X-ray Timing Experiment on the Rossi X-ray Timing Explorer satellite. In addition to the previously reported strong emission-line features produced by thermal plasmas, the broad-band spectrum has a high-energy "tail" that extends to energies at least as high as 120 keV. This tail may be described by a broken power law that has photon indices of Γ 1 = 1.8 +0.5 −0.6 and Γ 2 = 3.04 +0.15 −0.13 and a break energy of E b = 15.9 +0.3 −0.4 keV. We argue that the high-energy component, which dominates the spectrum above about 10 keV, is produced by synchrotron radiation from electrons that have energies up to at least 40 TeV. This conclusion supports the hypothesis that Galactic cosmic rays are accelerated predominantly in supernova remnants.

We report the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), of the first sub-millisecond oscillation found in a celestial X-ray source. The quasi-periodic oscillations (QPO) come from Sco X-1 and have a frequency of approximately 1100 Hz, amplitudes of 0.6-1.2% (rms) and are relatively coherent, with Q up to ∼10 2 . The frequency of the QPO increases with accretion rate, rising from 1050 to 1130 Hz when the source moves from top to bottom along 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 Hz normal/flaring branch QPO. We also report the discovery of QPO with a frequency near 800 Hz that occurs, simultaneously with the 1100 Hz QPO, in the upper normal branch. We discuss several possible interpretations, one involving a millisecond X-ray pulsar whose pulses we see reflected off accretion-flow inhomogeneities. Finally, we report the discovery of ∼45 Hz QPO, most prominent in the middle of the normal branch, which might be magnetospheric beat-frequency QPO.