Gamma-ray burst 230307A lit up the sky like a thunderclap: in seconds, it released more energy than the Sun will emit over billions of years. Three detectors—GECAM-B, GECAM-C, and Fermi/GBM—were looking in the right direction at that moment. Amid the chaos of gamma-ray photons, one instrument picked up a rhythm for 160 milliseconds: 909 beats per second. This rhythm was not noise. It was too sharp, too coherent to be a fluke.

What is this rhythm? Astrophysicists have long debated what remains after neutron star mergers: a black hole or an ultra-dense spinning top, a millisecond magnetar. The latter predicts a periodic signal: if the magnetar has a hot spot or an irregularity, it will blink like a lighthouse with each rotation. But spotting this beacon is nearly impossible: the jet, a narrow stream of plasma ejected by the star, initially shines straight at us, and its brightness swamps the weak modulation. Only when the jet turns and we begin to see its edges can the periodicity emerge from the noise. In 230307A, that happened for exactly 160 milliseconds.

The search was blind and meticulous. The researchers sliced the data into sliding windows of 100 ms with a step of 7 ms, split each window by energy, and in each of the 629,314 fragments looked for periodicity in the range 500–2500 Hz. They found a single peak at 909 Hz with an amplitude of 27.2% in the energy range 98–248 keV. The statistical significance, after corrections for multiple testing, was 4.2 sigma, corresponding to a chance probability of about 1.45×10⁻⁵. Three independent instruments confirmed the finding, and the signal amplitude increased with photon energy, as expected from a neutron star's magnetosphere.

This is the first direct evidence for a millisecond magnetar as the central engine of a long gamma-ray burst from a compact object merger. Previously, such signals were sought in short bursts, but only weak quasi-periodic oscillations were found, which could be dismissed as chance. Here, the coherence and the coincidence with the transition to high-latitude emission tie the periodicity to the jet geometry and the mechanism of magnetic energy dissipation. The ICMART model—internal collisions triggering magnetic reconnection and turbulence—gains strong support.

What's next? Searching for similar signals in archival data of bright gamma-ray bursts lasting more than a minute. Next-generation sensitive detectors will be able to register fainter modulations. Each new detection is a window into the first seconds of a magnetar's life: how it spins down, how its magnetic field interacts with the jet, how heavy elements are forged in the kilonova. The beacon flashes only for an instant, but now we know where to look.