Lense-Thirring Precession and QPOs in Low Mass X-Ray Binaries

We show in this Letter that relativistic dragging of inertial frames around fast rotating collapsed stars is substantial and can give rise to observable effects. We apply this to the kHz quasi periodic oscillations (QPOs) sources, low mass X-ray binaries (LMXRBs) containing an accreting neutron star. Within the beat frequency model, 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 in the innermost disk regions which 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 low mass X-ray binaries) arise from the same mechanism. Subject headings: accretion, accretion disks — relativity — pulsars: general — stars: neutron, rotation — X–rays: stars