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arXiv · 1305.0821

VLT/FORS2 observations of four high-luminosity ULX candidates

M. Heida·P. G. Jonker·M. A. P. Torres·T. P. Roberts·G. Miniutti·A. C. Fabian·E. M. Ratti
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Abstract

We obtained VLT/FORS2 spectra of the optical counterparts of four high-luminosity (L_X >= 10^40 erg/s) ULX candidates from the catalog of Walton (2011). 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 8 meter 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 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(F_X/F_opt) in the range of -1 to 1.

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M. Heida, P. G. Jonker, M. A. P. Torres, T. P. Roberts, G. Miniutti, A. C. Fabian, E. M. Ratti. 2013-05-03. VLT/FORS2 observations of four high-luminosity ULX candidates. https://doi.org/10.1093/mnras%2Fstt756

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