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 x 10(44) erg of R-MGF = 3.8(-3.1)(+4.0) x 10(5) 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.
Publication Source (Journal or Book title)
ASTROPHYSICAL JOURNAL LETTERS
Burns, E., Svinkin, D., Hurley, K., Wadiasingh, Z., Negro, M., Younes, G., Hamburg, R., Ridnaia, A., Cook, D., Cenko, S. B., Aloisi, R., Ashton, G., Baring, M., Briggs, M. S., Christensen, N., Frederiks, D., Goldstein, A., Hui, C. M., Kaplan, D. L., Kasliwal, M. M., Kocevski, D., Roberts, O. J., Savchenko, V., Tohuvavohu, A., Veres, P., & Wilson-Hodge, C. A. (2021). Identification of a Local Sample of Gamma-Ray Bursts Consistent with a Magnetar Giant Flare Origin. ASTROPHYSICAL JOURNAL LETTERS, 907 (2) https://doi.org/10.3847/2041-8213/abd8c8