VLT/FORS2 observations of four high-luminosity ULX candidates

TL;DR

This study used VLT/FORS2 spectroscopy to identify the true nature of four high-luminosity ULX candidates, confirming one as a genuine ULX and classifying the others as background AGN, thereby refining ULX candidate identification methods.

Contribution

First optical spectroscopic confirmation of the nature of four high-luminosity ULX candidates, distinguishing genuine ULXs from background AGN using VLT/FORS2 observations.

Findings

One ULX confirmed in an HII region of ESO 306-003.

Three sources identified as background AGN with specific redshifts.

High likelihood of background AGN presence in systems with certain X-ray to optical flux ratios.

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.