  The recent localization of some short-hard gamma ray bursts (GRBs) in
galaxies with low star formation rates has lent support to the suggestion that
these events result from compact object binary mergers. We discuss how new
simulations in general relativity are helping to identify the central engine of
short-hard GRBs. Motivated by our latest relativistic black hole-neutron star
merger calculations, we discuss a scenario in which these events may trigger
short-hard GRBs, and compare this model to competing relativistic models
involving binary neutron star mergers and the delayed collapse of hypermassive
neutron stars. Distinguishing features of these models may help guide future
GRB and gravitational wave observations to identify the nature of the sources.
