A Compact Binary Merger Model for the Short, Hard GRB 050509b

The first X-ray afterglow for a short (~30 ms), hard gamma-ray burst (GRB) was detected by Swift on 2005 May 9 (GRB 050509b). No optical or radio counterpart was identified in follow-up observations. The tentative association of the GRB with a nearby giant elliptical galaxy at redshift z = 0.2248 would imply a total energy release Eγ, iso ≈ 3 × 1048 ergs and that the progenitor had traveled several tens of kiloparsecs from its point of origin, in agreement with expectations linking these events to the final merger of compact binaries driven by gravitational wave emission. We model the dynamical merger of such a system and the time-dependent evolution of the accretion tori thus created. The resulting energetics, variability, and expected durations are consistent with GRB 050509b originating from the tidal disruption of a neutron star by a stellar mass black hole, or of the merger of two neutron stars followed by prompt gravitational collapse of the massive remnant. We discuss how the available γ-ray and X-ray data provide a probe for the nature of the relativistic ejecta and the surrounding medium.