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 are 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-1 and possibly in 4U 1608-52, the frequency separation between the QPO peaks decreases with increasing inferred mass accretion rate. We show that the currently available Rossi X-Ray Timing Explorer data are consistent with the hypothesis that the peak separations in all sources vary by amounts similar to the variation in Sco X-1. We discuss the implications for models of the kilohertz QPOs.