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 magnetospheresurrounding the accreting mass. We argue that such an interface can occur notonly with a magnetized neutron star but also sometimes with an unmagnetizedneutron star or a black hole. The gas at the magnetospheric interface isgenerally Rayleigh-Taylor unstable and may also be Kelvin-Helmholtz unstable.Because of these instabilities, modes with low azimuthal wavenumbers m areexpected to grow to large amplitude. It is proposed that the resultingnonaxisymmetric structures contribute to the high frequency quasi-periodicoscillations that have been seen in neutron-star and black-hole X-ray binaries.The mode oscillation frequencies are calculated to be approximately equal to m\Omega_m, where \Omega_m is the angular velocity of the accreting gas at themagnetospheric radius. Thus, mode frequencies should often be in theapproximate ratio 1:2:3, etc. If the pressure of the gas in the disk is notlarge, then the m = 1 mode will be stable. In this case, the mode frequenciesshould be in the approximate ratio 2:3, etc. There is some observationalevidence for such simple frequency ratios.Comment: 38 pages, including 8 figures, ApJ in pres