A peculiar low-luminosity short gamma-ray burst from a double neutron star merger progenitor

TL;DR

This paper analyzes a low-luminosity short gamma-ray burst associated with a neutron star merger, suggesting it originates from a structured jet viewed off-axis, and identifies a potential new population of similar faint GRBs.

Contribution

It presents evidence that some faint sGRBs are from neutron star mergers viewed at large angles, expanding understanding of GRB diversity and event rates.

Findings

The GRB had unusually low peak luminosity and spectral energy.

Estimated event rate density is comparable to neutron star merger rates.

Archival data suggests a new population of low-luminosity, nearby sGRBs.

Abstract

Double neutron star (DNS) merger events are promosing candidates of short Gamma-ray Burst (sGRB) progenitors as well as high-frequecy gravitational wave (GW) emitters. On August 17, 2017, such a coinciding event was detected by both the LIGO-Virgo gravitational wave detector network as GW170817 and Gamma-Ray Monitor on board NASA's {\it Fermi} Space Telescope as GRB 170817A. Here we show that the fluence and spectral peak energy of this sGRB fall into the lower portion of the distributions of known sGRBs. Its peak isotropic luminosity is abnormally low. The estimated event rate density above this luminosity is at least $190^{+440}_{-160} {\rm Gpc^{-3} \ yr^{-1}}$, which is close to but still below the DNS merger event rate density. This event likely originates from a structured jet viewed from a large viewing angle. There are similar faint soft GRBs in the {\it Fermi} archival data, a small fraction of which might belong to this new population of nearby, low-luminosity sGRBs.