Off‐Axis Properties of Short Gamma‐Ray Bursts

Based on recent models of relativistic jet formation by thermal energydeposition around black hole-torus systems, the relation between the on- andoff-axis appearance of short, hard gamma-ray bursts (GRBs) is discussed interms of energetics, duration, average Lorentz factor, and probability ofobservation, assuming that the central engines are remnants of binary neutronstar or neutron star-black hole mergers. As a consequence of the interactionwith the torus matter at the jet basis and the subsequent expansion of the jetsinto an extremely low-density environment, the collimated ultrarelativisticoutflows possess flat core profiles with only little variation ofradially-averaged properties, and are bounded by very steep lateral edges.Owing to the rapid decrease of the isotropic-equivalent energy near the jetedges, the probability of observing the lateral, lower Lorentz factor wings issignificantly reduced and most short GRBs should be seen with on-axis-likeproperties. Taking into account cosmological and viewing angle effects,theoretical predictions are made for the short-GRB distributions with redshiftz, fluence, and isotropic-equivalent energy. The observational data for shortbursts with determined redshifts are found to be compatible with thepredictions only if either the intrinsic GRB rate density drops rapidly at z>1,or a large number of events at z>1 are missed, implying that the subenergeticGRB 050509b was an extremely rare low-fluence event with detectable photon fluxonly because of its proximity and shortness. It appears unlikely that GRB050509b can be explained as an off-axis event. The detection of short GRBs withsmall Lorentz factors is statistically disfavored, suggesting a possible reasonfor the absence of soft short bursts in the duration-hardness diagram.Comment: 24 pages, 4 figures, accepted by The Astrophys. Journal; extended and revised for including referee comment