Effects of Radiation Forces on the Frequency of Gravitomagnetic Precession near Neutron Stars

Gravitomagnetic precession near neutron stars and black holes has receivedmuch recent attention, particularly as a possible explanation of 15--60 Hzquasi-periodic brightness oscillations (QPOs) from accreting neutron stars inlow-mass X-ray binaries, and of somewhat higher-frequency QPOs from accretingstellar-mass black holes. Previous analyses of this phenomenon have eitherignored radiation forces or assumed for simplicity that the radiation field isisotropic, and in particular that there is no variation of the radiation fieldwith angular distance from the rotational equatorial plane of the compactobject. However, in most realistic accretion geometries (e.g., those in whichthe accretion proceeds via a geometrically thin disk) the radiation fielddepends on latitude. Here we show that in this case radiation forces typicallyhave an important, even dominant, effect on the precession frequency of testparticles in orbits that are tilted with respect to the star's rotationalequator. Indeed, we find that even for accretion luminosities only a fewpercent of the Eddington critical luminosity, the precession frequency near aneutron star can be changed by factors of up to $\sim 10$. Radiation forcesmust therefore be included in analyses of precession frequencies near compactobjects, in such varied contexts as low-frequency QPOs, warp modes of disks,and trapped oscillation modes. We discuss specifically the impact of radiationforces on models of low-frequency QPOs involving gravitomagnetic precession,and show that such models are rendered much less plausible by the effects ofradiation forces.Comment: 15 pages LaTeX including three figures, submitted to Ap