THE BEAT-FREQUENCY INTERPRETATION OF KILOHERTZ QUASI-PERIODIC OSCILLATIONS IN NEUTRON-STAR LOW-MASS X-RAY BINARIES

Pairs of quasi-periodic oscillations (QPOs) at kilohertz frequencies a re a common phenomenon in several neutron-star low-mass X-ray binaries . The frequency separation of the QPO peaks in the pair appears to be constant in many sources and directly related to the neutron star spin frequency. However, in Sco X-l and possible in 4U 1608-52, the freque ncy separation between the QPO peaks decreases with increasing inferre d mass accretion rate. We show that the currently available Rossi X-Ra y Timing Explorer data an consistent with the hypothesis that the peak separations in all sources vary by amounts similar to the variation i n Sco X-l. We discuss the implications for models of the kilohertz QPO s.