Off‐Axis Properties of Short Gamma‐Ray Bursts

Abstract

Based on recent models of relativistic jet formation by thermal energy deposition around black hole-torus systems, the relation between the on- and off-axis appearance of short, hard gamma-ray bursts (GRBs) is discussed in terms of energetics, duration, average Lorentz factor, and probability of observation, assuming that the central engines are remnants of binary neutron star (NS+NS) or neutron star-black hole (NS+BH) mergers. As a consequence of the interaction with the torus matter at the jet base and the subsequent expansion of the jets into an extremely low density environment, the collimated ultrarelativistic outflows possess flat core profiles with only little variation of radially averaged properties and are bounded by very steep lateral edges. Owing to the rapid decrease of the isotropic equivalent energy near the jet edges, the probability of observing the lateral, lower Lorentz factor wings is significantly reduced, and most short GRBs should be seen with on-axis-like properties. Taking into account cosmological and viewing angle effects, theoretical predictions are made for the short-GRB distributions with redshift z, fluence, and isotropic equivalent energy. The observational data for short bursts with determined redshifts are found to be compatible with the predictions 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 subenergetic GRB 050509b was an extremely rare low-fluence event with detectable photon flux only because of its proximity and shortness. It appears unlikely that GRB 050509b can be explained as an off-axis event. The detection of short GRBs with small Lorentz factors is statistically disfavored, suggesting a possible reason for the absence of soft short bursts in the duration-hardness diagram.