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

Understand

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.