{
  "id": "1903.07527",
  "version": "v1",
  "published": "2019-03-18T16:08:35.000Z",
  "updated": "2019-03-18T16:08:35.000Z",
  "title": "The Energy-dependent $γ$-ray Morphology of the Crab Nebula Observed with the Fermi Large Area Telescope",
  "authors": [
    "Paul K. H. Yeung",
    "Dieter Horns"
  ],
  "comment": "Accepted for publication in ApJ",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "The Crab Nebula is a bright emitter of non-thermal radiation across the entire accessible range of wavelengths. The spatial and spectral structures of the synchrotron nebula are well-resolved from radio to hard X-ray emission. The un-pulsed emission at GeV to TeV energies is mostly produced via inverse-Compton scattering of energetic electrons with the synchrotron-emitted photons. The spatial structure observed at these energies provides insights into the distribution of electrons and indirectly constrains the so-far unknown structure of the magnetic field in the nebula. Analyzing the LAT data accumulated over $\\sim$9.1 years with a properly refined model for the Crab pulsar's spectrum, we determined the 68\\% containment radius ($R_{68}$) of the Crab Nebula to be $({0.0330\\pm0.0025_{stat}}{^{+0.0012}_{-0.0075}}_{sys})^\\circ$ (${1.98'\\pm0.15'_{stat}}{^{+0.07'}_{-0.45'}}_{sys}$) in the 5--500 GeV band. The estimated systematic uncertainty is based on two factors: (1) different analysis methods, morphological models and event types, and (2) the point-spread-function evaluated with observations of Mkn 421. When comparing the \\emph{Fermi} LAT and H.E.S.S. results on the spatial extension, we find evidence for an energy-dependent shrinking of the Crab Nebula's $\\gamma$-ray extension ($R_{68}\\propto E_\\mathrm{IC}^{-\\alpha}$ where $\\alpha=0.155\\pm0.035_{stat}{-0.037}_{sys}$).",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-03-18T16:08:35.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "fermi large area telescope",
      "crab nebula",
      "ray morphology",
      "energy-dependent",
      "crab pulsars spectrum"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}