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 , \buildrel > \over { \sim } 10 ^ { 41 } ergs s ^ { -1 } , 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 He ii ( \lambda 4686 Å ) line .
This line shows a large velocity dispersion of \simeq 410 km s ^ { -1 } , consistent with the existence of a compact ( < 5 AU ) highly-ionized accretion disc rotating around the central X-ray source .
The \sim 1.4 \times 10 ^ { 37 } ergs s ^ { -1 } luminosity of the He ii 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 He ii 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 .