Identification of a Local Sample of Gamma-Ray Bursts Consistent with a Magnetar Giant Flare Origin

TL;DR

This study identifies a local population of gamma-ray bursts likely originating from magnetar giant flares, revealing their rates, host galaxy associations, and implications for magnetar progenitors and repeating GRBs.

Contribution

It provides the first unambiguous identification of nearby extragalactic MGFs as a distinct GRB class, with estimated rates and host galaxy analysis supporting their origin from magnetars formed in core-collapse supernovae.

Findings

Four local short GRBs are consistent with magnetar giant flares.

The inferred volumetric rate of MGFs is approximately 3.8×10^5 Gpc^-3 yr^-1.

Properties suggest MGFs are a dominant source of extragalactic gamma-ray transients.

Abstract

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 contribute a third class of extragalactic GRBs. We report the unambiguous identification of a distinct population of 4 local ($<$5 Mpc) short GRBs, adding GRB 070222 to previously discussed events. While identified solely based on alignment to 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 non-detection in gravitational waves all point to an extragalactic MGF origin. Despite the small sample, the inferred volumetric rates for events above $4\times10^{44}$ erg of $R_{MGF}=3.8_{-3.1}^{+4.0}\times10^5$ Gpc$^{-3}$ yr$^{-1}$ place MGFs as 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.