Hydromagnetic waves in a superfluid neutron star with strong vortex pinning

TL;DR

This paper investigates how strong vortex pinning in a superfluid neutron star core affects hydromagnetic waves, with implications for magnetar oscillations and superfluid turbulence stability.

Contribution

It demonstrates that vortex pinning minimally impacts core Alfven waves and stabilizes the Glaberson instability, providing new insights into neutron star core dynamics.

Findings

Core Alfven waves are not significantly mass-loaded by neutrons despite strong pinning.

Decoupling 95% of core mass is needed to match QPO frequencies.

Vortex pinning stabilizes the Glaberson instability against superfluid turbulence.

Abstract

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.