Bardeen-Petterson Effect and Quasi-Periodic Oscillations in X-Ray Binaries

The Bardeen-Petterson effect around a rapidly-rotating compact object causes a tilted accretion disk to warp into the equatorial plane of the rotating body. Viscous forces cause the accretion flow to divide into two distinct regions - an inner aligned accretion disk and an outer tilted accretion disk. The transition between these two occurs at a characteristic radius that depends on the mass and angular momentum of the central object and possibly on the accretion rate through the disk. We propose that accreting material passing through the transition region may generate quasi-periodic brightness oscillations (QPOs) such as have been observed in a number of X-ray binaries. We show that this effect may be present in the black-hole X-ray binary GRO J1655-40. We also argue that the QPO frequency range predicted by this model is consistent with observed QPO frequencies in both black-hole and neutron-star low-mass X-ray binaries.