Cosmological gamma-ray bursts (GRBs) are known to arise from distinct progenitor channels: short GRBs mostly from neutron star mergers and long GRBs from a rare type of core-collapse supernova (CCSN) called collapsars. Highly magnetized neutron stars called magnetars also generate energetic, short-duration gamma-ray transients called magnetar giant flares (MGFs). Three have been observed from the Milky Way and its satellite galaxies, and they have long been suspected to constitute a third class of extragalactic GRBs. We report the unambiguous identification of a distinct population of four local (<5 Mpc) short GRBs, adding GRB 070222 to previously discussed events. While identified solely based on alignment with nearby star-forming galaxies, their rise time and isotropic energy release are independently inconsistent with the larger short GRB population at >99.9% confidence. These properties, the host galaxies, and nondetection in gravitational waves all point to an extragalactic MGF origin. Despite the small sample, the inferred volumetric rates for events above 4 10⁴⁴ erg of Gpc⁻³ yr⁻¹ make MGFs the dominant gamma-ray transient detected from extragalactic sources. As previously suggested, these rates imply that some magnetars produce multiple MGFs, providing a source of repeating GRBs. The rates and host galaxies favor common CCSN as key progenitors of magnetars.

已知宇宙伽马射线暴 (GRB) 来自不同的前身通道：短 GRB 主要来自中子星并合，而长 GRB 来自罕见类型的核心坍缩超新星 (CCSN)，称为坍缩星。被称为磁星的高度磁化的中子星也会产生称为磁星巨耀斑 (MGF) 的高能、短时伽马射线瞬变。在银河系及其卫星星系中已经观测到了三个，并且长期以来一直怀疑它们构成了第三类河外伽马暴。我们报告了对四个本地 (<5 Mpc) 短 GRB 的不同种群的明确识别，将 GRB 070222 添加到先前讨论的事件中。虽然仅根据与附近恒星形成星系的对齐来确定，它们的上升时间和各向同性能量释放与较大的短 GRB 群体独立不一致，置信度大于 99.9%。这些属性、宿主星系和引力波中的未探测都指向河外 MGF 起源。尽管样本很小，但对于 4 10 以上事件的推断体积速率⁴⁴ erg 的Gpc ^-3 yr ^-1使 MGF 成为从河外源检测到的主要伽马射线瞬变。如前所述，这些速率意味着一些磁星会产生多个 MGF，提供重复 GRB 的来源。速率和宿主星系有利于共同的 CCSN 作为磁星的主要祖先。