A magnetar giant flare in the nearby starburst galaxy M82

Abstract

Magnetar giant flares are rare explosive events releasing up to 10<SUP>47</SUP> erg in gamma rays in less than 1 second from young neutron stars with magnetic fields up to 10<SUP>15−16</SUP> G (refs. <SUP>1,2</SUP>). Only three such flares have been seen from magnetars in our Galaxy<SUP>3,4</SUP> and in the Large Magellanic Cloud<SUP>5</SUP> 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 10<SUP>46</SUP> erg s<SUP>−1</SUP>, 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<SUP>50−53</SUP> erg) events, originating in compact binary mergers<SUP>6</SUP>. A few short GRBs have been proposed<SUP>7-11</SUP>, 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. <SUP>12</SUP>). 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.