{
  "id": "1901.03776",
  "version": "v1",
  "published": "2019-01-12T00:46:23.000Z",
  "updated": "2019-01-12T00:46:23.000Z",
  "title": "Searching for the Donor Stars of ULX Pulsars",
  "authors": [
    "M. Heida",
    "F. A. Harrison",
    "M. Brightman",
    "F. Fürst",
    "D. Stern",
    "D. J. Walton"
  ],
  "comment": "10 pages, 3 figures, accepted for publication in ApJ",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We report on our search for the optical counterparts of two ultraluminous X-ray pulsars with known orbital periods, M82 X-2 and NGC 5907 X-1, in new and archival HST observations, in an effort to characterize the donor stars in these systems. We detect five near-infrared sources consistent with the position of M82 X-2 that are too bright to be single stars. We also detect seven sources in the WFC3/UVIS F336W image whose photometry matches that of 10-15 M$_\\odot$ stars turning off the main sequence. Such stars have densities consistent with the properties of the donor star of M82 X-2 as inferred from X-ray timing analysis, although it is also possible that the donor is a lower mass star below our detection limit or that there is a significant contribution from the accretion disc to the optical emission. We detect three candidate counterparts to NGC 5907 X-1 in the near-infrared. All of these are too bright to be the donor star of the ULX, which based on its orbital period is a red giant. The high background at the location of NGC 5907 X-1 precludes us from detecting this expected donor star. The recently discovered NGC 5907 ULX-2 also falls within the field of view of the near-infrared imaging; we detect four sources in the error circle, with photometry that matches AGB stars. The star suggested to be the counterpart of NGC 5907 ULX-2 by Pintore et al. (2018) falls outside our 2-$\\sigma$ error circle.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-01-12T00:46:23.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "donor star",
      "ulx pulsars",
      "error circle",
      "orbital period",
      "detect seven sources"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 10,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}