The identification of an optical counterpart to the super - Eddington X - ray source , NGC 5204 X - 1 3

We report the identification of a possible optical counterpart to the super-Eddington X-ray source NGC 5204 X-1. New CHANDRA data shows that the X-ray source is point-like, with a luminosity of 5.2 × 10 erg s (0.5 8 keV). It displays mediumand long-term X-ray variability in observations spanning a period of 20 years. The accurate CHANDRA position allows us to identify a blue optical continuum source (mv = 19.7) at the position of NGC 5204 X-1, using newly-obtained optical data from the INTEGRAL instrument on the William Herschel Telescope. The X-ray and optical source properties are consistent with the scenario in which we are observing the beamed X-ray emission of a high-mass X-ray binary in NGC 5204, composed of an O star with either a black hole or neutron star companion.