{
  "id": "1709.08319",
  "version": "v1",
  "published": "2017-09-25T04:58:22.000Z",
  "updated": "2017-09-25T04:58:22.000Z",
  "title": "Detection of gamma-ray emission from a galaxy cluster with Fermi Large Area Telescope",
  "authors": [
    "Shao-Qiang Xi",
    "Xiang-Yu Wang",
    "Yun-Feng Liang",
    "Fang-Kun Peng",
    "Rui-Zi Yang",
    "Ruo-Yu Liu"
  ],
  "comment": "7 pages, 3 figures and 3 tables, submitted on Sept. 24, 2017",
  "categories": [
    "astro-ph.HE",
    "astro-ph.CO",
    "astro-ph.GA"
  ],
  "abstract": "Many galaxy clusters have giant halos of non-thermal radio emission, indicating the presence of relativistic electrons in the clusters. Relativistic protons may also be accelerated by merger and/or accretion shocks in galaxy clusters. These cosmic-ray (CR) protons are expected to produce gamma-rays through inelastic collisions with thermal gas of the intra-cluster medium. Despite of intense efforts in searching for high-energy gamma-ray emission from galaxy clusters, conclusive evidence is still missing so far. Here we report the discovery of $\\ge100$ MeV gamma-rays from the Coma cluster at a confidence level of $\\sim6 \\sigma$ with the {\\it Fermi} Large Area Telescope (LAT). The result is obtained with the unbinned likelihood analysis of the nine years of {\\it Fermi}-LAT Pass 8 data. The gamma-ray emission shows an extended spatial morphology roughly coincident with the giant radio halo, with an apparent excess at the southwest of the cluster. The integral energy flux in the energy range of 0.1-300 GeV is about $3\\times 10^{-12}{\\rm \\ erg\\ cm^{-2}\\ s^{-1}}$. The spectrum of the gamma-ray emission is soft with a photon index of $\\simeq-2.7$. Interpreting the gamma-ray emission as arising from CR proton interaction, we find that the volume-averaged value of the CR to thermal pressure ratio in the Coma cluster is about $\\sim 2\\%$. Our results show that galaxy clusters are a new type of GeV gamma-ray sources, and they are probably also giant reservoirs of CR protons.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-09-25T04:58:22.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "galaxy cluster",
      "gamma-ray emission",
      "fermi large area telescope",
      "spatial morphology roughly coincident",
      "coma cluster"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 7,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}