{
  "id": "1510.07915",
  "version": "v1",
  "published": "2015-10-27T14:15:26.000Z",
  "updated": "2015-10-27T14:15:26.000Z",
  "title": "Optical counterparts of two ULXs in NGC5474 and NGC3627 (M66)",
  "authors": [
    "S. Avdan",
    "A. Vinokurov",
    "S. Fabrika",
    "K. Atapin",
    "H. Avdan",
    "A. Akyuz",
    "O. Sholukhova",
    "N. Aksaker",
    "A. Valeev"
  ],
  "comment": "5 pages, 3 figures, to appear in MNRAS Letters",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We identified two optical counterparts of brightest ultraluminous X-ray Sources (ULXs) in galaxies NGC5474 and NGC3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 ($M_V \\approx -4.5$) and 25.9 ($M_V \\approx -4.2$), respectively. NGC5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has $L_X \\approx 1.6 \\times 10^{40}$ erg/s, the spectrum is best fitted by an absorbed power-law model with a photon index $\\Gamma \\approx 0.94$. M66 X-1 varies in X-rays with a factor of ~2.5, its maximal luminosity being $2.0 \\times 10^{40}$ erg/s with $\\Gamma \\approx 1.7$. Optical spectroscopy of the NGC5474 X-1 has shown a blue spectrum, which however was contaminated by a nearby star of 23 mag, but the counterpart has a redder spectrum. Among other objects captured by the slit are a background emission-line galaxy (z=0.359) and a new young cluster of NGC5474. We find that these two ULXs have largest X-ray-to-optical ratios of $L_X/L_{opt}$ ~ 7000 for NGC5474 X-1 (in its bright state) and 8000 for M66 X-1 both with the faintest optical counterparts ever measured. Probably their optical emission originates from the donor star. If they have super-Eddington accretion discs with stellar-mass black holes, they may also have the lowest mass accretion rates among ULXs such as in M81 X-6 and NGC1313 X-1.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2015-10-27T14:15:26.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "optical counterparts",
      "lowest mass accretion rates",
      "bright state",
      "brightest ultraluminous x-ray sources",
      "hubble space telescope images"
    ],
    "publication": {
      "doi": "10.1093/mnrasl/slv155"
    },
    "note": {
      "typesetting": "TeX",
      "pages": 5,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 1400985
    }
  }
}