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

Context. GRB 211211A was accompanied by a kilonova, implying a merger origin for the event. A precursor flare, modulated by quasi-periodic oscillations at approximate to 22 Hz, was also seen approximate to 1 s prior to the main emission. Aims. 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. Methods. Torsional mode properties were computed for sequences of slowly rotating, magnetised neutron stars in general relativity. Results. The approximate to 22 Hz modulations in the precursor matches that of nodeless,l = 2 torsional modes for a wide variety of stellar parameters. Global or discrete Alfven modes are also viable candidates.

Automated summary

The study suggests that the precursor flare of GRB 211211A event may have resulted from the resonant shattering of one star's crust prior to coalescence, generating seismic aftershocks and low-frequency torsional modes. This interpretation is supported by the computed torsional mode properties, showing similarities between the precursor and specific torsional modes. Potential candidates for the modulations in the precursor include global or discrete Alfven modes.