Torsional nodeless vibrations of quaking neutron star restored by combined forces of shear elastic and magnetic field stresses

TL;DR

This paper models torsional nodeless vibrations in neutron stars, showing that combined elastic and magnetic forces can explain observed QPOs during magnetar flares.

Contribution

It introduces a magneto-solid-mechanics model for neutron star vibrations considering both elastic and magnetic forces, applied to interpret QPOs in magnetar flares.

Findings

QPOs can be explained by torsional nodeless vibrations

The model matches observed frequencies of SGR flares

Magnetic and elastic forces jointly restore vibrations

Abstract

Within the framework of Newtonian magneto-solid-mechanics, relying on equations appropriate for a perfectly conducting elastic continuous medium threaded by a uniform magnetic field, the asteroseismic model of a neutron star undergoing axisymmetric global torsional nodeless vibrations under the combined action of Hooke's elastic and Lorentz magnetic forces is considered with emphasis on a toroidal Alfv\'en mode of differentially rotational vibrations about the dipole magnetic moment axis of the star. The obtained spectral equation for frequency is applied to $\ell$-pole identification of quasi-periodic oscillations (QPOs) of X-ray flux during the giant flares of SGR 1806-20 and SGR 1900+14. Our calculations suggest that detected QPOs can be consistently interpreted, within the framework of this model, as produced by global torsional nodeless vibrations of quaking magnetar if they are considered to be restored by the joint action of bulk forces of shear elastic and magnetic field stresses.