Quasi-periodic oscillations in precursor flares via seismic aftershocks from resonant shattering

TL;DR

This paper proposes that the precursor flare in GRB 211211A resulted from resonant crust shattering in a neutron star, with observed oscillations explained by torsional or Alfvén modes, linking gamma-ray burst precursors to starquake phenomena.

Contribution

It introduces a model connecting precursor flares to seismic aftershocks and torsional modes in neutron stars, supported by calculations matching observed oscillation frequencies.

Findings

Precursor flare oscillations at ~22 Hz match torsional modes in neutron stars.

Resonant crust shattering can produce seismic aftershocks explaining precursor modulations.

Model supports crustal vibrations as origin of gamma-ray burst precursors.

Abstract

GRB 211211A was accompanied by a kilonova, implying a merger origin for the event. A precursor flare, modulated by quasi-periodic oscillations at $\approx 22\, \mbox{Hz}$, was also seen $\approx 1\, \mbox{s}$ prior to the main emission. It is suggested here that the precursor resulted from the resonant shattering of one star's crust prior to coalescence. Seismic aftershocks and low-frequency torsional modes may emanate from the rupture site in this case, explaining the underlying oscillations. This interpretation is directly analogous to proposals for giant flares from soft gamma repeaters, where oscillations at similar frequencies have been observed, involving starquakes followed by crustal vibrations. Torsional mode properties were computed for sequences of slowly rotating, magnetised neutron stars in general relativity. The $\approx 22\, \mbox{Hz}$ modulations in the precursor matches that of nodeless, $\ell =2$ torsional modes for a wide variety of stellar parameters. Global or discrete Alfv\'en modes are also viable candidates.