2001MNRAS.325L...7R

We report the identification of a possible optical counterpart to the super-Eddington X-ray source NGC 5204 X-1. New Chandra data show that the X-ray source is point-like, with a luminosity of 5.2x10³⁹erg.s^–1 (0.5-8keV). It displays medium- and long-term X-ray variability in observations spanning a period of 20yr. 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.

¹A note on taxonomy: we refer to the class of L_X>10³⁹erg.s^–1 extra-nuclear sources as SESs throughout this Letter. This class of object is alternatively referred to as SLS, IXOs or ultra-luminous compact X-ray sources (ULXs) in the literature, although the last acronym refers in particular to objects suspected to be powered by accretion.

²The positional error radius is a combination of the ≲1arcsec Chandra positional error and a similar error on the INTEGRAL position, which in practice encompasses the central fibre plus its six immediate neighbours.

³We note that, although O-star spectra generally display absorption features, these are low-contrast features and we do not have the signal-to-noise ratio to distinguish them in the current optical spectrum.