Quasi-periodic oscillations from Rayleigh-Taylor and Kelvin-Helmholtz instability at a disk-magnetosphere interface

We consider the interface between an accretion disk and a magnetosphere surrounding the accreting mass. We argue that such an interface can occur not only with a magnetized neutron star but also sometimes with an unmagnetized neutron star or a black hole. The gas at the magnetospheric interface is generally Rayleigh-Taylor unstable and may also be Kelvin-Helmholtz unstable. Because of these instabilities, modes with low azimuthal wavenumbers m are expected to grow to large amplitude. It is proposed that the resulting nonaxisymmetric structures contribute to the high-frequency quasi-periodic oscillations that have been seen in neutron star and black hole X-ray binaries. The mode oscillation frequencies are calculated to be approximately equal to mOmega(m), where Omega(m) is the angular velocity of the accreting gas at the magnetospheric radius. Thus, mode frequencies should often be in the approximate ratio 1 : 2 : 3, etc. If the pressure of the gas in the disk is not large, then the m = 1 mode will be stable. In this case, the mode frequencies should be in the approximate ratio 2 : 3, etc. There is some observational evidence for such simple frequency ratios.