Optical study of the hyper-luminous X-ray source 2XMM J011942.7+032421

We present the identification and characterization of the optical counterpart to 2XMM J011942.7+032421, one of the most luminous and distant ultra-luminous X-ray sources (ULXs). The counterpart is located near a star forming region in a spiral arm of the galaxy NGC 470 with u, g, and r magnitudes of 21.53, 21.69, and 21.71 mags, respectively. The luminosity of the counterpart is much larger than that of a single O-type star, indicating that it may be a stellar cluster. Our optical spectroscopic observations confirm the association of the X-ray source and the optical counterpart with its host galaxy NGC 470, which validates the high, > 10^41 erg/s, X-ray luminosity of the source. Its optical spectrum is embedded with numerous emission lines, including H recombination lines, metallic forbidden lines and more notably the high-ionization HeII (lambda 4686 A) line. This line shows a large velocity dispersion of $\simeq$ 410 \kms, consistent with the existence of a compact (< 5 AU) highly-ionized accretion disc rotating around the central X-ray source. The 1.4 x 10^37 erg/s luminosity of the HeII line emission makes the source one of the most luminous ULXs in the emission of that line. This, together with the high X-ray luminosity and the large velocity dispersion of the HeII emission, suggests that the source is an ideal candidate for more extensive follow-up observations for understanding the nature of hyper-luminous X-ray sources, a more luminous subgroup of ULXs and more likely candidates for intermediate-mass black holes.