{
  "id": "1404.0617",
  "version": "v2",
  "published": "2014-04-02T17:08:52.000Z",
  "updated": "2014-05-29T12:17:12.000Z",
  "title": "X-ray diagnostics of chemical composition of the accretion disk and donor star in UCXBs II: XMM-Newton observations",
  "authors": [
    "Filippos Koliopanos",
    "Marat Gilfanov",
    "Lars Bildsten",
    "Maria Diaz Trigo"
  ],
  "comment": "Accepted by MNRAS. 11 pages, 4 figures",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We search for the Fe K${\\alpha}$ line in spectra of Ultra Compact X-ray Binaries (UCXBs). For this purpose we have analyzed XMM-Newton observations of five confirmed UCXBs. We find that the object 2S 0918-549 - whose optical spectrum bears tentative signatures of a C/O accretion disk - is devoid of any emission features in the 6-7 keV range, with an upper limit of less than 10 eV for the equivalent width (EW) of the iron line. 4U 1916-05 - whose optical spectrum is consistent with reflection from a He-rich accretion disk - exhibits a bright broad iron emission line. This behavior is in agreement with the theoretical predictions presented in Koliopanos, Gilfanov and Bildsten (2013). Namely, we expect strong suppression of the Fe K${\\alpha}$ emission line in spectra originating in moderately bright (LogLx less than $\\approx$ 37.5) UCXBs with C/O or O/Ne/Mg-rich donors. On the other hand the EW of the iron line in spectra from UCXBs with He-rich donors is expected to retain its nominal value of $\\approx$ 100 eV. Our analysis also reveals a strong Fe K${\\alpha}$ line in the spectrum of 4U 0614+091. This detection points towards a He-rich donor and seems to be at odds with the source's classification as C/O-rich. Nevertheless, a He-rich donor would explain the bursting activity reported for this system. Lastly, based on our theoretical predictions, we attribute the lack of a strong iron emission line - in the two remaining UCXB sources in our sample (XTE J1807-294, 4U 0513-40) - as an indication of a C/O or O/Ne/Mg white dwarf donor. From the upper limits of the Fe K${\\alpha}$ line EW in 4U 0513-40, 2S 0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-iron ratio,O/Fe$\\ge$ 10$\\times$[O/Fe]$_{\\odot}$.",
  "revisions": [
    {
      "version": "v2",
      "updated": "2014-05-29T12:17:12.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "accretion disk",
      "xmm-newton observations",
      "x-ray diagnostics",
      "donor star",
      "chemical composition"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 11,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 1288305
    }
  }
}