Coherent magneto-elastic oscillations in superfluid magnetars

TL;DR

This study investigates how superfluidity influences magneto-elastic oscillations in highly magnetized neutron stars, revealing two main oscillation types and their potential link to observed magnetar flare phenomena.

Contribution

It introduces a two-dimensional magnetohydrodynamical-elastic simulation approach that accounts for superfluid effects, identifying two distinct oscillation groups and providing frequency fits.

Findings

Confirmed existence of continuum and global oscillations in superfluid magnetars.

Global oscillations may explain observed quasi-periodic oscillations in magnetar flares.

No crustal shear modes found at typical magnetar magnetic field strengths.

Abstract

We study the effect of superfluidity on torsional oscillations of highly magnetised neutron stars (magnetars) with a microphysical equation of state by means of two-dimensional, magnetohydrodynamical- elastic simulations. The superfluid properties of the neutrons in the neutron star core are treated in a parametric way in which we effectively decouple part of the core matter from the oscillations. Our simulations confirm the existence of two groups of oscillations, namely continuum oscillations that are confined to the neutron star core and are of Alfv\'enic character, and global oscillations with constant phase and that are of mixed magneto-elastic type. The latter might explain the quasi-periodic oscillations observed in magnetar giant flares, since they do not suffer from the additional damping mechanism due to phase mixing, contrary to what happens for continuum oscillations. However, we cannot prove rigorously that the coherent oscillations with constant phase are normal modes. Moreover, we find no crustal shear modes for the magnetic field strengths typical for magnetars.We provide fits to our numerical simulations that give the oscillation frequencies as functions of magnetic field strength and proton fraction in the core.