{
  "id": "1901.10536",
  "version": "v1",
  "published": "2019-01-29T20:46:59.000Z",
  "updated": "2019-01-29T20:46:59.000Z",
  "title": "Detection of non-thermal hard X-ray emission from the \"Fermi bubble\" in an external galaxy",
  "authors": [
    "Jiang-Tao Li",
    "Edmund Hodges-Kluck",
    "Yelena Stein",
    "Joel N. Bregman",
    "Judith A. Irwin",
    "Ralf-Jurgen Dettmar"
  ],
  "comment": "7 pages, 3 figures, ApJ in press",
  "categories": [
    "astro-ph.HE",
    "astro-ph.GA"
  ],
  "abstract": "We report new Chandra hard X-ray ($>2\\rm~keV$) and JVLA C-band observations of the nuclear superbubble of NGC 3079, an analog of the \"Fermi bubble\" in our Milky Way. We detect extended hard X-ray emission on the SW side of the galactic nucleus with coherent multi-wavelength features in radio, H$\\alpha$, and soft X-ray. The hard X-ray feature has a cone shape with possibly a weak cap, forming a bubble-like structure with a diameter of $\\sim1.1\\rm~kpc$. A similar extended feature, however, is not detected on the NE side, which is brighter in all other wavelengths such as radio, H$\\alpha$, and soft X-ray. Scattered photons from the nuclear region or other nearby point-like X-ray bright sources, inverse Compton emission from cosmic ray electrons via interaction with the cosmic microwave background, or any individually faint stellar X-ray source populations, cannot explain the extended hard X-ray emission on the SW side and the strongly NE/SW asymmetry. A synchrotron emission model, plus a thermal component accounting for the excess at $\\sim1\\rm~keV$, can well characterize the broadband radio/hard X-ray spectra. The broadband synchrotron spectra do not show any significant cutoff, and even possibly slightly flatten at higher energy. This rules out a loss-limited scenario in the acceleration of the cosmic ray electrons in or around this superbubble. As the first detection of kpc-scale extended hard X-ray emission associated with a galactic nuclear superbubble, the spatial and spectral properties of the multi-wavelength emissions indicate that the cosmic ray leptons responsible for the broad-band synchrotron emission from the SW bubble must be accelerated in situ, instead of transported from the nuclear region of the galaxy.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-01-29T20:46:59.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "extended hard x-ray emission",
      "non-thermal hard x-ray emission",
      "stellar x-ray source populations",
      "faint stellar x-ray source",
      "fermi bubble"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 7,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}