{
  "id": "1710.06421",
  "version": "v1",
  "published": "2017-10-17T17:54:33.000Z",
  "updated": "2017-10-17T17:54:33.000Z",
  "title": "Off-Axis Emission of Short GRB Jets from Double Neutron Star Mergers and GRB 170817A",
  "authors": [
    "Jonathan Granot",
    "Ramandeep Gill",
    "Dafne Guetta",
    "Fabio De Colle"
  ],
  "comment": "14 pages, 10 figures",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "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 $n\\sim10^{-2}-10^{-1}~{\\rm cm}^{-3}$.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-10-17T17:54:33.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "double neutron star mergers",
      "short grb jets",
      "grb 170817a",
      "off-axis emission",
      "relativistic jet viewed off-axis"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 14,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}