Lense-Thirring Precession and Quasi-periodic Oscillations in Low-Mass X-Ray Binaries

Relativistic dragging of inertial frames around fast-rotating collapsed stars is substantial and can give rise to observable effects. We consider kHz quasi-periodic oscillation (QPO) sources, low-mass X-ray binaries (LMXRBs) containing an accreting neutron star. Within beat frequency models, both the Keplerian frequency of the innermost region of the accretion disk (~0.3-1.2 kHz) and the neutron star spin frequency (~0.3-0.4 kHz) are directly observed. From these, the Lense-Thirring precession frequency (tens of Hz) of the same material as gives rise to the kHz QPOs is determined within a factor of ~4, depending on the neutron star equation of state. The classical contribution from neutron star oblateness decreases the precession frequency slightly. The broad peaks at frequencies ~20-40 Hz in the power spectra of the "Atoll" sources 4U 1728-34, 4U 0614+091, and KS 1731-260 and their variations with the higher kHz QPO frequency are well matched by Lense-Thirring precession of material in the innermost disk region. We also suggest that the ~15-50 Hz horizontal branch QPOs of GX 5-1 and GX 17+2 (and likewise other Z-type LMXRBs) arise from the same mechanism.