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 (similar to 0.3-1.2 kHz) and the neutron star spin frequency (similar to 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 similar to 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 similar to 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 similar to 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.