Implications for the origin of short gamma-ray bursts from their observed positions around their host galaxies

TL;DR

This paper analyzes the positions of short gamma-ray bursts relative to their host galaxies, supporting the idea that most originate from merging compact binaries, with some possibly formed dynamically in globular clusters.

Contribution

It compares observed burst offsets with predictions from binary merger models, highlighting the role of galaxy type and suggesting some bursts may originate from globular cluster mergers.

Findings

Most offsets match binary merger models

Elliptical galaxy offsets are dominated by galaxy size

Some bursts may originate from globular cluster mergers

Abstract

We present the observed offsets of short-duration gamma-ray bursts (SGRBs) from their putative host galaxies and compare them to the expected distributions of merging compact object binaries, given the observed properties of the hosts. We find that for all but one burst in our sample the offsets are consistent with this model. For the case of bursts with massive elliptical host galaxies, the circular velocities of the hosts' haloes exceed the natal velocities of almost all our compact object binaries. Hence the extents of the predicted offset distributions for elliptical galaxies are determined largely by their spatial extents. In contrast, for spiral hosts the galactic rotation velocities are smaller than typical binary natal velocities and the predicted burst offset distributions are more extended than the galaxies. One SGRB, 060502B, apparently has a large radial offset that is inconsistent with an origin in a merging galactic compact binary. Although it is plausible that the host of GRB 060502B is mis-identified, our results show that the large offset is compatible with a scenario where at least a few per cent of SGRBs are created by the merger of compact binaries that form dynamically in globular clusters.