Off-Axis Emission of Short GRB Jets from Double Neutron Star Mergers and GRB 170817A

Abstract

The short-duration ($\lesssim 2$s) GRB 170817A in the nearby ($D=40$Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) 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 and radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. Comparison to kilonova models favors the formation of a short-lived ($\sim 1$s) hypermassive NS, which is also supported by the $\Delta t\approx 1.74$s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (8.9days) and radio (16.4days) emission, together with the low isotropic equivalent $\gamma$-ray energy output ($E_{\gamma ,\mathrm{iso}}\approx 5\times 10^{46}$erg), strongly suggest emission from a narrow relativistic jet viewed off-axis. 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 lightcurves expected in such a case to the X-ray to radio emission from GRB 170817A. We fit an afterglow off-axis jet model to the X-ray and radio data and find that the observations are explained by a viewing angle ${\theta }_{\mathrm{obs}}\approx 16^{\circ }-26^{\circ }$, GRB jet energy $E\sim 10^{48.5}-10^{49.5}\mathrm{erg}$, and external density $n\sim 10^{-5}-10^{-1}{\mathrm{cm}}^{-3}$ for a ${\xi }_e\sim 0.1$ non-thermal electron acceleration efficiency.

14 pages, 10 figures; Minor changes made to text; Fig. 10 now shows results from least-squares fit instead of MCMC simulation; updated references; Submitted to MNRAS

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