Short gamma-ray bursts from SGR giant flares and neutron star mergers: two populations are better than one

TL;DR

This paper investigates whether short gamma-ray bursts originate from one or two different populations, finding that a bimodal luminosity function with two distinct populations better explains observed data and local burst rates.

Contribution

It introduces a bimodal luminosity function model for short gamma-ray bursts, supporting the existence of two distinct progenitor populations: neutron star mergers and SGR giant flares.

Findings

A bimodal luminosity function is necessary to fit the data.

Lower luminosity population matches properties of SGR giant flares.

Predicted local burst numbers align with previous estimates.

Abstract

There is increasing evidence of a local population of short duration Gamma-ray Bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible Luminosity Functions (LF) for both neutron star binary mergers and giant flares from Soft Gamma Repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall BATSE sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertainties in the intrinsic rates, we find that at least a bimodal LF consisting of lower and higher luminosity populations is required to reproduce both the overall BATSE sGRB number counts and a local burst distribution. Furthermore, the best fit parameters of the lower luminosity population agree well with the known properties of SGR giant flares, and the predicted numbers are sufficient to account for previous estimates of the local sGRB population.