Off-axis emission of short GRB jets from double neutron star mergers and GRB 170817A

The short-duration (less than or similar to 2s) GRB 170817A in the nearby (D approximate to 40 Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave detection of a binary neutron-star (NS-NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray to radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactivedecay powered kilonova-like emission. The late onset of the X-ray (8.9 d) and radio (16.4d) afterglow emission, together with the low isotropic equivalent gamma-ray energy output (E-gamma ,E- iso approximate to 5 x 10(46) erg), strongly suggest emission from a narrow relativistic jet viewed off-axis, initially dominated by low-energy material along our line of sight and gradually overtaken by the more energetic parts of the jet near its core. Here, we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet's core, and we compare the afterglow light curves expected in such a case to the X-ray, optical, and radio emission from GRB 170817A. We fit the data using a simulation-based off-axis relativistic jet afterglow model featuring an initially top-hat jet, and find a satisfactory fit for a viewing angle to initial jet half-opening angle ratio of theta(obs)/theta(0) approximate to 3, or theta(obs) approximate to 17 degrees(theta(0)/0.1).