Forced oscillations in relativistic accretion disks and QPOs

We explore the idea that the high-frequency quasi-periodic oscillations (kHz QPOs) observed in Low Mass X-ray Binaries (LMXBs) may be explained as a resonant coupling between the neutron star spin and epicyclic modes of accretion disk oscillations. We propose a new model for these QPOs based on forced oscillations induced in the disk due to a stellar asymmetric rotating gravitational or magnetic field. Particles evolving in this field are subject to three kinds of resonances: a corotation resonance, a Lindblad resonance due to a driving force, and a parametric resonance due to the time varying epicyclic frequencies. These results are deduced from a linear stability analysis and are extended by means of 2D numerical simulations. The simulations are performed for the Newtonian gravitational potential, as well as for a pseudo-general relativistic potential, which enables us to explore the behavior of the resonances around both rotating neutron stars and black holes. When applied to a typical neutron star, we found that the strongest response occurs when the frequency difference equals either the spin frequency (for "slow rotators") or half of it (for "fast rotators"). The two main excited modes may both be connected to vertical oscillations of the disk. We emphasize that strong gravity is not needed to excite the modes.