Magneto-elastic torsional oscillations of magnetars

TL;DR

This paper develops a numerical model to study magneto-elastic oscillations in magnetars, revealing how strong magnetic fields and crustal elasticity influence observed quasi-periodic oscillations.

Contribution

It extends a general-relativistic MHD code to include elasticity, enabling detailed analysis of magnetar oscillations and their spectral features.

Findings

Crustal shear oscillations are accurately recovered without magnetic fields.

Magnetic fields significantly modify Alfvén oscillations, creating distinct QPO families.

QPOs split into lower, edge, and upper groups with specific spectral characteristics.

Abstract

We extend a general-relativistic ideal magneto-hydrodynamical code to include the effects of elasticity. Using this numerical tool we analyse the magneto-elastic oscillations of highly magnetised neutron stars (magnetars). In simulations without magnetic field we are able to recover the purely crustal shear oscillations within an accuracy of about a few per cent. For dipole magnetic fields between 5 x 10^13 and 10^15 G the Alfv\'en oscillations become modified substantially by the presence of the crust. Those quasi-periodic oscillations (QPOs) split into three families: Lower QPOs near the equator, Edge QPOs related to the last open field line and Upper QPOs at larger distance from the equator. Edge QPOs are called so because they are related to an edge in the corresponding Alfv\'en continuum. The Upper QPOs are of the same kind, while the Lower QPOs are turning-point QPOs, related to a turning point in the continuous spectrum.