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

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 x 10^39 erg/s (0.5 - 8 keV). It displays medium- and 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 (m_v = 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.