Magnetar Oscillations II: spectral method

The seismological dynamics of magnetars is largely determined by a strong hydro-magnetic coupling between the solid crust and the fluid core. In this paper we set up a spectral computational framework in which the magnetars motion is decomposed into a series of basis functions which are associated with the crust and core vibrational eigenmodes. A general-relativistic formalism is presented for evaluation of the core Alfven modes in the magnetic-flux coordinates, as well for eigenmode computation of a strongly magnetized crust of finite thickness. By considering coupling of the crustal modes to the continuum of Alfven modes in the core, we construct a fully relativistic dynamical model of the magnetar which allows: i) Fast and long simulations without numerical dissipation. ii) Very fine sampling of the stellar structure. We find that the presence of strong magnetic field in the crust results in localizing of some high-frequency crustal elasto-magnetic modes with the radial number n>1 to the regions of the crust where the field is nearly horizontal. While the hydro-magnetic coupling of these localized modes to the Alfven continuum in the core is reduced, their energy is drained on a time-scale much less than 1 second. Therefore the puzzle of the observed QPOs with frequencies larger than 600 Hz still stands.

Hydromagnetic waves in a superfluid neutron star with strong vortex pinning

Neutron-star cores may be hosts of a unique mixture of a neutron superfluid and a proton superconductor. Compelling theoretical arguments have been presented over the years that if the proton superconductor is of type II, than the superconductor fluxtubes and superfluid vortices should be strongly coupled and hence the vortices should be pinned to the proton-electron plasma in the core. We explore the effect of this pinning on the hydromagnetic waves in the core, and discuss 2 astrophysical applications of our results: 1. We show that even in the case of strong pinning, the core Alfven waves thought to be responsible for the low-frequency magnetar quasi-periodic oscillations (QPO) are not significantly mass-loaded by the neutrons. The decoupling of about 0.95 of the core mass from the Alfven waves is in fact required in order to explain the QPO frequencies, for simple magnetic geometries and for magnetic fields not greater than 10^{15} Gauss. 2. We show that in the case of strong vortex pinning, hydromagnetic stresses exert stabilizing influence on the Glaberson instability, which has recently been proposed as a potential source of superfluid turbulence in neutron stars.