{
  "id": "1706.01481",
  "version": "v1",
  "published": "2017-06-05T18:12:49.000Z",
  "updated": "2017-06-05T18:12:49.000Z",
  "title": "The Bright and the Slow -- GRBs 100724B \\& 160509A with high-energy cutoffs at $\\lesssim100\\;$MeV",
  "authors": [
    "G. Vianello",
    "R. Gill",
    "J. Granot",
    "N. Omodei",
    "J. Cohen-Tanugi",
    "F. Longo"
  ],
  "comment": "Submitted to ApJ",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the brightest Gamma-Ray Bursts (GRBs) observed by Fermi at $\\lesssim{\\rm MeV}$ energies but surprisingly faint at $\\gtrsim100\\;$MeV energies. Time-resolved spectroscopy reveals a sharp high-energy cutoff at energies $E_c\\sim20-60\\;$MeV for GRB~100724B and $E_c\\sim80-150\\;$MeV for GRB~160509A. We first characterize phenomenologically the cutoff and its time evolution. We then fit the data to two models where the high-energy cutoff arises from intrinsic opacity to pair production within the source ($\\tau_{\\gamma\\gamma}$): (i) a Band spectrum with $\\tau_{\\gamma\\gamma}$ from the internal-shocks motivated model of Granot et al. (2008), and (ii) the photospheric model of Gill & Thompson (2014). Alternative explanations for the cutoff, such as an intrinsic cutoff in the emitting electron energy distribution, appear to be less natural. Both models provide a good fit to the data with very reasonable physical parameters, providing a direct estimate of bulk Lorentz factors in the range $\\Gamma\\sim 100-400$, on the lower end of what is generally observed in Fermi GRBs. Surprisingly, their lower cutoff energies $E_c$ compared to other Fermi-LAT GRBs arise not predominantly from the lower Lorentz factors, but also at a comparable level from differences in variability time, luminosity, and high-energy photon index. Finally, particularly low $E_c$ values may prevent detection by Fermi-LAT, thus introducing a bias in the Fermi-LAT GRB sample against GRBs with low Lorentz factors or variability times.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-06-05T18:12:49.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "high-energy cutoff",
      "grbs 100724b",
      "variability time",
      "lower cutoff energies",
      "brightest gamma-ray bursts"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}