The Peculiar Precursor of a Gamma-Ray Burst from a Binary Merger Involving a Magnetar

TL;DR

This paper reports a peculiar electromagnetic precursor observed before a gamma-ray burst associated with a binary merger, providing evidence of magnetar activity prior to the merger event, which challenges existing models of binary neutron star evolution.

Contribution

It presents the first observational evidence of an electromagnetic precursor indicating magnetar activity before a binary merger, with detailed spectral and temporal analysis.

Findings

Detection of a 0.2 s precursor with a non-thermal spectrum.

Identification of a 22 Hz QPO candidate in the precursor.

Implication of magnetar activity in the merger process.

Abstract

The milestone discovery of GW 170817-GRB 170817A-AT 2017gfo has shown that gravitational wave (GW) could be produced during the merger of neutron star-neutron star/black hole and that in electromagnetic (EM) wave a gamma-ray burst (GRB) and a kilonova (KN) are generated in sequence after the merger. Observationally, however, EM property before the merger phase is still unclear. Here we report a peculiar precursor in a KN-associated long-duration GRB 211211A, providing evidence of the EM before the merger. This precursor lasts $\sim$ 0.2 s, and the waiting time between the precursor and the main burst is $\sim$ 1 s, comparable to that between GW 170817 and GRB 170817A. The spectrum of the precursor could be well fit with a non-thermal cutoff power-law model instead of a blackbody. Especially, a $\sim$22 Hz Quasi-Periodic Oscillation candidate ($\sim 3\sigma$) is detected in the precursor. These temporal and spectral properties indicate that this precursor is probably produced by a catastrophic flare accompanying with magnetoelastic or crustal oscillations of a magnetar in binary compact merger. The strong magnetic field of the magnetar can also account for the prolonged duration of GRB 211211A. However, it poses a challenge to reconcile the rather short lifetime of a magnetar with the rather long spiraling time of a binary neutron star system only by the GW radiation before merger.