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D. Markovic, F. Lamb (U. Illinois)
We have investigated whether general relativistic effects may be responsible for some of the quasi-periodic X-ray brightness oscillations (QPOs) with frequencies ~ 20--300~Hz observed in low-mass binary systems containing accreting neutron stars and black hole candidates. In particular, we have computed the motions of accreting gas in the strong gravitational fields near such objects and have explored possible mechanisms for producing X-ray flux oscillations. We have discovered a family of global gravitomagnetic (Lense-Thirring) warping modes of the inner accretion disk that have precession frequencies ranging up to the single-particle gravitomagnetic precession frequency at the inner edge of the disk, which is ~30~Hz if the disk extends inward to the innermost stable circular orbit around a compact object of solar mass with dimensionless angular momentum cJ/GM2 ~0.2. The highest-frequency warping modes are very localized spiral corrugations of the inner disk and are weakly damped, with Q values ~ 2--50. Precession of regions of enhanced viscous dissipation or modulation of the accretion flow by the precession may produce observable periodic variation of the X-ray flux. Detectable effects might also be produced if the gas in the inner disk breaks up into a collection of distinct clumps. We have analyzed the dynamics of such clumps as well as the conditions required for their formation and survival on time scales long enough to produce oscillations with the coherence observed in X-ray binaries.
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