Quasi-periodic oscillations during magnetar giant flares in the strangeon star model

TL;DR

This study models torsional oscillations in strangeon stars to explain observed QPOs during magnetar giant flares, highlighting the role of shear modulus and ocean-crust interface modes.

Contribution

It provides a systematic analysis of torsional oscillation frequencies in strangeon star models, incorporating magnetic fields and relativistic effects, to explain high- and low-frequency QPOs.

Findings

High-frequency QPOs explained by large shear modulus of strangeon stars.

Low-frequency QPOs interpreted via ocean-crust interface modes.

Quark stars unlikely to account for all observed QPOs.

Abstract

Soft gamma-ray repeaters (SGRs) are widely understood as slowly rotating isolated neutron stars. Their generally large spin-down rates, high magnetic fields, and strong outburst energies render them different from ordinary pulsars. In a few giant flares (GFs) and short bursts of SGRs, high-confidence quasi-periodic oscillations (QPOs) were observed. Although remaining an open question, many theoretical studies suggest that the torsional oscillations caused by starquakes could explain QPOs. Motivated by this scenario, we systematically investigate torsional oscillation frequencies based on the strangeon-star (SS) model with various values of harmonic indices and overtones. To characterize the strong-repulsive interaction at short distances and the non-relativistic nature of strangeons, a phenomenological Lennard-Jones model is adopted. We show that, attributing to the large shear modulus of SSs, our results explain well the high-frequency QPOs ($\gtrsim 150\,\mathrm{Hz}$) during the GFs. The low-frequency QPOs ($\lesssim 150\,\mathrm{Hz}$) can also be interpreted when the ocean-crust interface modes are included. We also discuss possible effects of the magnetic field on the torsional mode frequencies. Considering realistic models with general-relativistic corrections and magnetic fields, we further calculate torsional oscillation frequencies for quark stars. We show that it would be difficult for quark stars to explain all QPOs in GFs. Our work advances the understanding of the nature of QPOs and magnetar asteroseismology.