Coupled polar-axial magnetar oscillations

TL;DR

This study investigates how strong magnetic fields in neutron stars couple axial and polar oscillations, transforming the spectrum from a continuum to discrete modes, with implications for crustal oscillations and energy loss.

Contribution

It demonstrates that magnetic coupling alters the oscillation spectrum, turning continua into discrete modes and affecting crustal oscillation longevity in magnetar models.

Findings

Axial spectrum loses continuum character due to magnetic coupling.

Crustal frequencies become long-lived oscillations.

Resonance at continuum edges can lead to energy loss.

Abstract

We study coupled axial and polar axisymmetric oscillations of a neutron star endowed with a strong magnetic field, having both poloidal and toroidal components. The toroidal component of the magnetic field is driving the coupling between the polar and axial oscillations. The star is composed of a fluid core as well as a solid crust. Using a two dimensional general relativistic simulation and a magnetic field B = 10^16 G, we study the change in the polar and axial spectrum caused by the coupling. We find that the axial spectrum suffers a dramatic change in its nature, losing its continuum character. In fact, we find that only the 'edges' of the continua survive, generating a discrete spectrum. As a consequence the crustal frequencies, that in our previous simulation could be absorbed by the continua, if they were embedded inside it, are now long living oscillations. They may lose their energy only in the very special case that they are in resonance with the 'edges' of the continua.