Ultra-luminous X-Ray sources are extra-nuclear point sources in external galaxies with L _ { X } = 10 ^ { 39 } – 10 ^ { 41 } erg/s and are among the most poorly understood X-ray sources .
To help understand their nature , we are trying to identify their optical counterparts by combining images from the Hubble Space Telescope and the Chandra Observatory .
Here we report upon the optical counterpart for the ULX in NGC 5204 , which has average X-ray luminosity of \sim 3 \times 10 ^ { 39 } erg/s and has varied by a factor of 50 % over the last 10 years .
A unique optical counterpart to this ULX is found by carefully comparing the Chandra ACIS images and HST WFPC2 and ACS/HRC images .
The spectral energy distribution and the HST/STIS FUV spectrum of this object show that it is a B0 Ib supergiant star with peculiarities , including the \lambda 1240 N V emission line that is uncommon in B stellar spectra but has been predicted for X-ray illuminated accretion disks and seen in some X-ray binaries .
Study of its FUV spectrum leads to a binary model for this ULX in which the B0 Ib supergiant is overflowing its Roche Lobe and accreting onto the compact primary , probably a black hole .
This picture predicts an orbital period of \sim 10 days for different black hole mass , which can be tested by future observations .