{
  "id": "1909.06393",
  "version": "v1",
  "published": "2019-09-13T18:13:52.000Z",
  "updated": "2019-09-13T18:13:52.000Z",
  "title": "Two years of non-thermal emission from the binary neutron star merger GW170817: rapid fading of the jet afterglow and first constraints on the kilonova fastest ejecta",
  "authors": [
    "A. Hajela",
    "R. Margutti",
    "K. D. Alexander",
    "A. Kathirgamaraju",
    "A. Baldeschi",
    "C. Guidorzi",
    "D. Giannios",
    "W. Fong",
    "Y. Wu",
    "A. MacFadyen",
    "A. Paggi",
    "E. Berger",
    "P. K. Blanchard",
    "R. Chornock",
    "D. L. Coppejans",
    "P. S. Cowperthwaite",
    "T. Eftekhari",
    "S. Gomez",
    "G. Hosseinzadeh",
    "T. Laskar",
    "B. D. Metzger",
    "M. Nicholl",
    "K. Paterson",
    "D. Radice",
    "L. Sironi",
    "G. Terreran",
    "V. A. Villar",
    "P. K. G. Williams",
    "X. Xie",
    "J. Zrake"
  ],
  "comment": "Submitted to ApJ, 13 pages, 6 figures",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay $F_{\\nu}\\propto t^{-1.95\\pm0.15}$ and a simple power-law spectrum $F_{\\nu}\\propto \\nu^{-0.575\\pm0.007}$. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find $n\\le 9.6 \\times 10^{-3}\\,\\rm{cm^{-3}}$. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy $E_0 = 1.5_{-1.1}^{+3.6}\\times 10^{49}\\,\\rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}\\times10^{52}\\, \\rm{erg}$), jet opening angle $\\theta_{0}= 5.9^{+1.0}_{-0.7}\\,\\rm{deg}$ with characteristic Lorentz factor $\\Gamma_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} \\times 10^{-3}\\, \\rm{cm^{-3}}$ and viewed $\\theta_{obs} = 30.4^{+4.0}_{-3.4}\\, \\rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are %loosely constrained to $\\epsilon_{\\rm{e}} = 0.18_{-0.13}^{+0.30}$ and $\\epsilon_{\\rm{B}}=2.3_{-2.2}^{+16.0} \\times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\\le2.5\\sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{KN}\\propto (\\Gamma\\beta)^{-\\alpha}$ into the environment, finding that shallow stratification indexes $\\alpha\\le6$ are disfavored.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-09-13T18:13:52.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "binary neutron star merger gw170817",
      "kilonova fastest ejecta",
      "non-thermal emission",
      "jet afterglow",
      "first constraints"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 13,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}