2024Natur.629...58M

Magnetar giant flares are rare explosive events releasing up to 10⁴⁷ erg in gamma rays in less than 1 second from young neutron stars with magnetic fields up to 10^15–16 G (refs. ). Only three such flares have been seen from magnetars in our Galaxy and in the Large Magellanic Cloud in roughly 50 years. This small sample can be enlarged by the discovery of extragalactic events, as for a fraction of a second giant flares reach luminosities above 10⁴⁶ erg s^–1, which makes them visible up to a few tens of megaparsecs. However, at these distances they are difficult to distinguish from short gamma-ray bursts (GRBs); much more distant and energetic (10^50–53 erg) events, originating in compact binary mergers. A few short GRBs have been proposed, with different amounts of confidence, as candidate giant magnetar flares in nearby galaxies. Here we report observations of GRB 231115A, positionally coincident with the starburst galaxy M82 (ref. ). Its spectral properties, along with the length of the burst, the limits on its X-ray and optical counterparts obtained within a few hours, and the lack of a gravitational wave signal, unambiguously qualify this burst as a giant flare from a magnetar in M82.