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

The short-duration (≲ 2 s) GRB 170817A in the nearby (D ≈ 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 radioactive-decay powered kilonova-like emission. The late onset of the X-ray (8.9 d) and radio (16.4 d) afterglow emission, together with the low isotropic equivalent γ-ray energy output (E_{γ, iso} ≈ 5 × 10⁴⁶ 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 θ_obs/θ₀ ≈ 3, or θ_obs ≈ 17°(θ₀/0.1).