A new instability of accretion disks around magnetic compact stars

Aperiodic variability and quasi-periodic oscillations (QPOs) are observed f rom accretion disks orbiting white dwarfs, neutron stars, and black holes, suggesting that the flow is universally broken up into discrete blobs. We c onsider the interaction of these blobs with the magnetic held of a compact, accreting star, where diamagnetic blobs suffer a drag. We show that when t he magnetic moment is not aligned with the spin axis, the resulting force i s pulsed, and this can lead to resonance with the oscillation of the blobs around the equatorial plane; a resonance condition where energy is effectiv ely pumped into nonequatorial motions is then derived. We show that the sam e resonance condition applies for the quadrupolar component of the magnetic field. We discuss the conditions of applicability of this result, showing that they are quite wide. We also show that realistic complications, such a s chaotic magnetic fields, buoyancy, radiation pressure, evaporation, Kelvi n-Helmholtz instability, and shear stresses due to differential rotation do not affect our results. In accreting neutron stars with millisecond period s, we show that this instability leads to Lense-Thirring precession of the blobs and that damping by viscosity can be neglected.