{
  "id": "2210.13052",
  "version": "v1",
  "published": "2022-10-24T09:16:44.000Z",
  "updated": "2022-10-24T09:16:44.000Z",
  "title": "Measurement of the $\\sim 10^{-16}$~Gauss inter-galactic magnetic field with high energy emission of GRB 221009A",
  "authors": [
    "Zi-Qing Xia",
    "Yun Wang",
    "Qiang Yuan",
    "Yi-Zhong Fan"
  ],
  "comment": "5 pages, 3 figures",
  "categories": [
    "astro-ph.HE",
    "astro-ph.CO"
  ],
  "abstract": "The fast evolving TeV-PeV transients and their delayed GeV-TeV cascade emission in principle server as an ideal probe of the inter-galactic magnetic fields which are hard to be measured by other methods. Very recently, LHASSO has detected the very high energy emission of the extraordinary powerful GRB 221009A up to $\\sim 18$ TeV within $\\sim 2000$ s after the burst trigger. Here we report the detection of a $\\sim 400$ GeV photon, without accompanying prominent $\\gamma$ rays down to $\\sim 2$ GeV, by Fermi-LAT in the direction of GRB 221009A at about 0.4 days after the burst. Such a hard spectrum is unexpected in the inverse Compton radiation of the electrons accelerated by the external forward shock. Instead, the inverse Compton scattering of the $e^\\pm$ pairs, produced from the cascade of the early primary $\\sim 20$ TeV $\\gamma$ rays, off the diffuse far-infrared and microwave backgrounds can generate $\\gamma$ rays up to $\\sim 400$ GeV with a rather hard low energy spectrum. We infer that an inter-galactic magnetic field strength of $B_{\\rm IGMF}\\sim 10^{-16}$ Gauss can naturally account for the arrival time of the $\\sim 400$ GeV photon. Such a $B_{\\rm IGMF}$ is comparable to the limits set by the statistical studies of the high energy emission of TeV blazars.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2022-10-24T09:16:44.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "high energy emission",
      "gauss inter-galactic magnetic field",
      "grb 221009a",
      "measurement",
      "gev photon"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 5,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}