Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries - a Review

This paper provides a review of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries recently discovered with NASA's Rossi X-ray Timing Explorer. It contains two tables that are up to date up to October 1st, 1997 with information about all kHz QPO phenomena so far reported. In the most straightforward interpretations, kHz QPO provide direct estimates of the millisecond spin rate of neutron stars in low-mass X-ray binaries and of the Kepler frequency at the inner edge of the accretion disk. If this is so, the accretion disk must penetrate nearly to the bottom of the potential well of these neutron stars. The kHz QPO then directly constrain the mass and radius of these neutron stars, and thereby the equation of state of high-density matter, and potentially allow the detection of major strong-field general-relativistic effects such as the existence of an innermost stable orbit, and frame dragging.