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 10¹⁵⁻¹⁶ G (refs. ^1,2). Only three such flares have been seen from magnetars in our Galaxy^3,4 and in the Large Magellanic Cloud⁵ 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 (10⁵⁰⁻⁵³ erg) events, originating in compact binary mergers⁶. A few short GRBs have been proposed^7,8,9,10,11, 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. ¹²). 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.

磁星巨耀斑是一种罕见的爆炸事件，在不到 1 秒的时间内从磁场高达 10 ^{15−16 G 的年轻中子星释放出高达 10 47}  erg 的伽马射线 （参考文献^1,2）。大约 50 年来，我们的银河系^3,4和大麦哲伦星云⁵中的磁星只观测到了 3 次这样的耀斑。这个小样本可以通过河外事件的发现来放大，因为在不到一秒的时间内，巨型耀斑的光度就达到了 10 ⁴⁶  erg s ^-1以上，这使得它们的可见度高达几十兆秒差距。然而，在这些距离上，它们很难与短伽马射线暴（GRB）区分开来。更遥远和更有活力（10 ⁵⁰⁻⁵³  erg）的事件，起源于紧凑的二元合并⁶。一些短伽玛暴被提出^7,8,9,10,11，具有不同的置信度，作为附近星系中的候选巨磁星耀斑。在这里，我们报告了 GRB 231115A 的观测结果，其位置与星暴星系 M82 重合（参考文献¹²）。它的光谱特性，连同爆发的长度、几个小时内获得的 X 射线和光学对应物的限制，以及缺乏引力波信号，明确地将这次爆发视为来自 M82 磁星的巨大耀斑。