Magneto-elastic oscillations and the damping of crustal shear modes in magnetars

TL;DR

This study models magneto-elastic oscillations in magnetars, showing crustal shear modes are rapidly damped by resonant absorption, and suggests Alfvén QPOs are a more viable explanation for observed oscillations if the magnetic field is sufficiently strong.

Contribution

It provides a realistic model demonstrating rapid damping of crustal shear modes and supports Alfvén QPOs as the explanation for observed magnetar oscillations at high magnetic fields.

Findings

Crustal shear oscillations are damped within 0.2 seconds.

Alfvén QPOs dominate at magnetic fields above 10^14 G.

Crustal shear modes cannot explain low-frequency QPOs.

Abstract

In a realistic model of magneto-elastic oscillations in magnetars, we find that crustal shear oscillations, often invoked as an explanation of quasi-periodic oscillations (QPOs) seen after giant flares in soft gamma-ray repeaters (SGRs), are damped by resonant absorption on timescales of at most 0.2s, for a lower limit on the dipole magnetic field strength of 5 x 10^13 G. At higher magnetic field strengths (typical in magnetars) the damping timescale is even shorter, as anticipated by earlier toy-models. We have investigated a range of equations of state and masses and if magnetars are dominated by a dipole magnetic field, our findings exclude torsional shear oscillations of the crust from explaining the observed low-frequency QPOs. In contrast, we find that the Alfv\'en QPO model is a viable explanation of observed QPOs, if the dipole magnetic field strength exceeds a minimum strength of about several times 10^14 G to 10^15 G. Then, Alfv\'en QPOs are no longer confined to the fluid core, but completely dominate in the crust region and have a maximum amplitude at the surface of the star.