Short-duration gamma-ray bursts are bright flashes of gamma-rays from extra-galactic sources. They are believed to arise in the merger of a compact binary, consisting of two neutron stars (NS-NS) or a neutron star and a black hole (BH-NS). Such a merger occurs following the emission of gravitational radiation by the binary, which spirals together as a consequence. Gravitational waves from the more nearby short gamma-ray bursts may be seen by the Advanced LIGO detector currently under construction.

Many short-duration gamma-ray bursts are seen outside their host galaxies, and the compact binary model provides a natural explanation of this observation. Neutron stars are observed to have large spatial velocities, which implies that they receive a velocity kick at birth. These kicks are similar to the escape speeds of typical galaxies, which can eject the compact binary from its host galaxy.

Using a binary population synthesis calculation we predicted the birth velocities of a large sample of NS-NS and NS-BH binaries, and predicted the expected offsets from the observed host galaxies of short gamma-ray bursts. The resulting distributions match the observations quite well, but a few bursts are found at too large radii from their hosts. We suggest that these may be the result of dynamical compact binary formation in globular clusters.

For more details, see the our paper on the topic (Church et al. 2011, published in MNRAS), or email Ross Church and Melvyn B. Davies.