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.
Highlights
The merger of a compact binary consisting of neutron stars (NSs) or black holes (BHs) is a prime model for the origin of the short-duration gamma-ray bursts (SGRBs)
This is unsurprising as such binaries are typically rather massive and strongly bound at the time of the kick and, as our results show, the properties of our sample of double compact object (DCO) binaries are relatively insensitive to the natal kick chosen for the first supernova
The offsets of all but one short GRB are found to be consistent with this picture
Summary
The merger of a compact binary consisting of neutron stars (NSs) or black holes (BHs) is a prime model for the origin of the short-duration gamma-ray bursts (SGRBs). The sample has been building up slowly and we are only approaching a stage where it is possible to compare the observed offsets with predictions for DCO binaries. We build on previous work that modelled the offset distributions of DCO mergers Bloom, Sigurdsson & Pols 1999; Fryer, Woosley & Hartmann 1999) by using the observed properties of the host galaxies to predict offset distributions on a host-by-host basis. We take the observed sample of SGRBs that have identified hosts and model the production and galactic trajectories of DCO binaries inside those hosts.
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