Quasi-periodic oscillations: Resonant shear Alfvén waves in neutron star magnetospheres

¹ Theoretical Physics Institute, Department of Physics, University of Alberta, Edmonton, AB, T6G 2J1, Canada e-mail: vrezania@phys.ualberta.ca
² Institute for Advanced Studies in Basic Sciences, Zanjan 45195, Iran

Received: 5 August 2004
Accepted: 19 February 2005

Abstract

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 Alfvén waves in a region of enhanced density gradients, the region where accretion material flows along the magnetic field lines in the magnetosphere (see Fig.1). 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 (a weaker field, farther from the star), to approximately kHz frequencies (a stronger field, ~ 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.