2025NatAs...9.1701C

The product of a compact star merger is usually hypothesized to be a hyperaccreting black hole, typically resulting in a gamma-ray burst (GRB) with a duration shorter than 2 s. However, recent observations of GRB 211211A and GRB 230307A, both arising from compact star mergers, challenge this model due to their minute-long durations. The data from both events are consistent with having a nascent, rapidly spinning, highly magnetized neutron star (a millisecond magnetar) as the merger product and GRB engine, but a smoking gun signature is still missing. Here we report strong but not yet conclusive evidence for the detection of a 909-Hz gamma-ray periodic signal during a brief time window of GRB 230307A, which is consistent with the rotation frequency of such a millisecond magnetar. Notably, the periodic signal appeared for only 160 ms at an epoch coinciding with the transition epoch when the jet emission from the GRB central engine ceased and when the delayed emission from high latitudes started. If this signal is real, the temporal and spectral features of this gamma-ray periodicity can be consistently interpreted as asymmetric mini-jet emission from a dissipating Poynting-flux-dominated jet, as revealed by the energy-dependent light curve data of this burst.