2013MNRAS.433..681H

We obtained Very Large Telescope/FOcal Reducer and low dispersion Spectrograph spectra of the optical counterparts of four high-luminosity (LX ≥ 1040ergs-1) Ultraluminous X-ray sources (ULX) candidates from the catalogue of Walton et al. We first determined accurate positions for the X-ray sources from archival Chandra observations and identified counterparts in archival optical observations that are sufficiently bright for spectroscopy with an 8m telescope. From the spectra we determine the redshifts to the optical counterparts and emission line ratios. One of the candidate ULXs, in the spiral galaxy ESO 306-003, appears to be a bona fide ULX in an Hii region. The other three sources, near the elliptical galaxies NGC 533 and NGC 741 and in the ring galaxy AM 0644-741, turn out to be background active galactic nuclei (AGN) with redshifts of 1.85, 0.88 or 1.75 and 1.40, respectively. Our findings confirm the trend of a high probability of finding background AGN for systems with a ratio of log(FX/Fopt) in the range of -1-1.