RAPID X-RAY VARIABILITY OF NEUTRON-STARS IN LOW-MASS BINARY-SYSTEMS

The dramatic discovery with the Rossi X-Ray Timing Explorer satellite of remarkably coherent similar to 300-1200 Hz oscillations in the X-ra y brightness of some sixteen neutron stars in low-mass binary systems has spurred theoretical modeling of these oscillations and investigati on of their implications for the neutron stars and accretion flows in these systems. High-frequency oscillations are observed both during th ermonuclear X-ray bursts and during intervals of accretion-powered emi ssion and appear to be a characteristic feature of disk-accreting neut ron stars with weak magnetic fields. In this review we focus on the hi gh-frequency quasi-periodic oscillations (QPOs) seen in the accretion- powered emission. We first summarize the key properties of these kiloh ertz QPOs and then describe briefly the models that have been proposed to explain them. The existing evidence strongly favors beat-frequency models. We mention several of the difficulties encountered in applyin g the magnetospheric beat-frequency model to the kilohertz QPOs. The m ost fully developed and successful model is the sonic-point beat-frequ ency model. We describe the work on this model in some detail. We then discuss observations that could help to distinguish between models. W e conclude by noting some of the ways in which study of the kilohertz QPOs may advance our understanding of dense matter and strong gravitat ional fields.