ON THE MAGNETOSPHERIC BEAT-FREQUENCY AND LENSE-THIRRING INTERPRETATIONS OF THE HORIZONTAL-BRANCH OSCILLATION IN THE Z SOURCES

摘要

Three types of quasi-periodic oscillations (QPOs) have been discovered so far in the persistent emis- sion of the most luminous neutron star low-mass X-ray binaries, the Z sources: ~ l0 - 60 Hz horizontal- branch and ~6 -20 Hz normal/flaring-branch oscillations and ~200 - l200 Hz kilohertz QPOs, which usually occur in pairs. Here we study the horizontal-branch oscillations and the two simultaneous kilo- hertz QPOs, which were discovered using the Rossi X-Ray Timing Explorer, comparing their properties in five Z sources with the predictions of the magnetospheric beat-frequency and Lense-Thirring preces- sion models. We find that the variation of the horizontal-branch oscillation frequency with accretion rate predicted by the magnetospheric beat-frequency model for a purely dipolar stellar magnetic field and a radiation-pressure-dominated inner accretion disk is consistent with the observed variation. The model predicts a universal relation between the horizontal-branch oscillation, stellar spin, and upper kilohertz QPO frequencies that agrees with the data on five Z sources. The model implies that the neutron stars in the Z sources are near magnetic spin equilibrium, that their magnetic field strengths are ~ 10~9-10~10 G, and that the critical fastness parameter for these sources is >~ 0.8. If the frequency of the upper kilo- hertz QPO is an orbital frequency in the accretion disk, the magnetospheric beat-frequency model requires that a small fraction of the gas in the disk does not couple strongly to the stellar magnetic field at 3-4 stellar radii but instead drifts slowly