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We present the identification and characterization of the optical counterpartto 2XMM J011942.7+032421, one of the most luminous and distant ultra-luminousX-ray sources (ULXs). The counterpart is located near a star forming region ina 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 largerthan 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-raysource and the optical counterpart with its host galaxy NGC 470, whichvalidates the high, > 10^41 erg/s, X-ray luminosity of the source. Its opticalspectrum is embedded with numerous emission lines, including H recombinationlines, 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-ionizedaccretion disc rotating around the central X-ray source. The 1.4 x 10^37 erg/sluminosity of the HeII line emission makes the source one of the most luminousULXs in the emission of that line. This, together with the high X-rayluminosity and the large velocity dispersion of the HeII emission, suggeststhat the source is an ideal candidate for more extensive follow-up observationsfor understanding the nature of hyper-luminous X-ray sources, a more luminoussubgroup of ULXs and more likely candidates for intermediate-mass black holes.

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