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Giant flares, short explosive events releasing up to 10$^{47}$ erg of energyin the gamma-ray band in less than one second, are the most spectacularmanifestation of magnetars, young neutron stars powered by a very strongmagnetic field, 10$^{14-15}$ G in the magnetosphere and possibly higher in thestar interior. The rate of occurrence of these rare flares is poorlyconstrained, as only three have been seen from three different magnetars in theMilky Way and in the Large Magellanic Cloud in about 50 years since thebeginning of gamma-ray astronomy. This sample can be enlarged by the discoveryof extragalactic events, since for a fraction of a second giant flares reachpeak luminosities above 10$^{46}$ erg/s, which makes them visible by currentinstruments up to a few tens of Mpc. However, at these distances they appearsimilar to, and difficult to distinguish from, regular short gamma-ray bursts(GRBs). The latter are much more energetic events, 10$^{50-53}$ erg, producedby compact binary mergers and originating at much larger distances. Indeed,only a few short GRBs have been proposed, with different levels of confidence,as magnetar giant flare candidates in nearby galaxies. Here we report thediscovery of a short GRB positionally coincident with the central region of thestarburst galaxy M82. Its spectral and timing properties, together with thelimits on its X-ray and optical counterparts obtained a few hours after theevent and the lack of an associated gravitational wave signal, qualify withhigh confidence this event as a giant flare from a magnetar in M82.

A magnetar giant flare in the nearby starburst galaxy M82