The peculiar precursor of a gamma-ray burst from a binary merger involving a magnetar

Abstract The milestone discovery of GW 170817-GRB 170817A-AT 2017gfo1 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 ∼ 0.2 s, and the waiting time between the precursor and the main burst is ∼ 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 (CPL) model instead of a blackbody. Especially, an Quasi-Periodic Oscillation (QPO) with frequency of ∼22 Hz is most likely 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 merger2. 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 magnetar3 with the rather long spiraling time of a binary neutron star system only by the GW radiation before merger.