The Rossi X-Ray Timing Explorer
  RXTE has been operating for nearly 2 years and is planning the third. The spacecraft performance has been good and the three instruments are operating well. Observations have been made of the range of targets suitable for RXTE, including such different objects as accreting neutron stars and black holes, stellar flares, and supernova remnants. The goals of studying high time resolution and broad energy range and optimizing multi-wavelength participation are yielding important results. Oscillations found from low-mass X-ray binaries probably are signatures of the spin of the neutron stars and of the shortest orbital periods around the neutron stars. These are constraining neutron star parameters. Oscillation and spectral results from black hole candidates bring into the realm of possibility the possibilities of measuring the spins of the black holes and using X-ray data to test predictions of gravitation theory. Multi-wavelength observations are leading to identification of the locations of the X-ray emission regions and, in the case of the micro-quasars, to understanding of the mechanisms for jet formation. Recently faster observing response than originally planned has made possible some RXTE contributions to identification of gamma-ray bursts. 
J. H. Swank