A bright γ-ray flare interpreted as a giant magnetar flare in NGC 253

Soft gamma-ray repeaters exhibit bursting emission in hard X-rays and soft gamma-rays. During the active phase, they emit random short (milliseconds to several seconds long), hard-X-ray bursts, with peak luminosities(1) of 10(36) to 10(43) erg per second. Occasionally, a giant flare with an energy of around 10(44) to 10(46) erg is emitted(2). These phenomena are thought to arise from neutron stars with extremely high magnetic fields (10(14) to 10(15) gauss), called magnetars(1,3,4). A portion of the second-long initial pulse of a giant flare in some respects mimics short gamma-ray bursts(5,6), which have recently been identified as resulting from the merger of two neutron stars accompanied by gravitational-wave emission(7). Two gamma-ray bursts, GRB 051103 and GRB 070201, have been associated with giant flares(2,8-11). Here we report observations of the gamma-ray burst GRB 200415A, which we localized to a 20-square-arcmin region of the starburst galaxy NGC 253, located about 3.5 million parsecs away. The burst had a sharp, millisecond-scale hard spectrum in the initial pulse, which was followed by steady fading and softening over 0.2 seconds. The energy released (roughly 1.3 x 10(46) erg) is similar to that of the superflare(5,12,13) from the Galactic soft gamma-ray repeater SGR 1806-20 (roughly 2.3 x 10(46) erg). We argue that GRB 200415A is a giant flare from a magnetar in NGC 253.The gamma-ray burst GRB 200415A is probably a giant flare emitted from a magnetar in the nearby starburst galaxy NGC 253.