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.