Different progenitors of short hard gamma-ray bursts

We consider the spatial offsets of short hard gamma-ray bursts (SHBs) from their host galaxies. We show that all SHBs with extended-duration soft emission components lie very close to their hosts. We suggest that neutron star–black hole binary mergers offer a natural explanation for the properties of this extended-duration/low-offset group. SHBs with large offsets have no observed extended emission components and are less likely to have an optically detected afterglow, properties consistent with neutron star–neutron star binary mergers occurring in low-density environments.