Magnetar oscillations in the presence of a crust

Abstract

We study the axisymmetric perturbations of neutron stars endowed with a strong magnetic field (magnetars), considering the coupled oscillations of the fluid core with the solid crust. We recover discrete oscillations based mainly on the crust and a continuum in the core. We also confirm the presence of 'discrete Alfvén modes' in the gap between two contiguous continua and, in addition, we can resolve some of them also inside the continua. Our results can explain both the lower and the higher observed quasi-periodical oscillations (QPOs) in SGR 1806-20 and SGR 1900+14 and put constraints on the mass, radius and crust thickness of the two magnetars. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.