http://arxiv.org/abs/1710.06421
The short-duration ($\lesssim2\;$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 ($\sim1\;$s) hypermassive NS, which is also supported by the $\Delta t\approx1.74\;$s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (9$\;$days) and radio (17$\;$days) emission, together with the low isotropic equivalent $\gamma$-ray energy output ($E_{\rm\gamma,iso}\approx5\times10^{46}\;$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 using a maximum likelihood analysis, and find that the data favor a viewing angle $\theta_{\rm obs}\approx37^\circ-42^\circ$, GRB jet energy $E\sim10^{48}-10^{49}~{\rm erg}$, and external density $n\sim10^{-2}-10^{-1}~{\rm cm}^{-3}$.
J. Granot, R. Gill, D. Guetta, et. al.
Wed, 18 Oct 2017
6/62
Comments: 14 pages, 10 figures