Quasi-periodic oscillations: Resonant shear Alfv`en waves in neutron star magnetospheres

Abstract

In this paper we propose a new model for quasi-periodic oscillations (QPOs) based on oscillating magnetohydrodynamic modes in neutron star magnetospheres. We argue that the interaction of the accretion disk with the magnetosphere can excite resonant shear Alfv`en waves in a region of enhanced density gradients, the region where accretion material flows along the magnetic field lines in the magnetosphere. We demonstrate that depending on the distance of this region from the star and the magnetic field strength, the frequency of the field line resonance can range from several Hz (weaker field, farther from star), to approximately kHz frequencies (stronger field, ~ 2-10 stellar radii from the star). We show that such oscillations are able to significantly modulate inflow of matter from the high density region toward the star's surface, and possibly produce the observed X-ray spectrum. In addition, we show that the observed 2:3 frequency ratio of QPOs is a natural result of our model.