The optical counterpart of an ultraluminous X-ray source in NGC 5204

Ultraluminous X-ray (ULX) sources are extranuclear point sources in external galaxies with L-X=10(39)-10(41) ergs s(-1) 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 (HST) and the Chandra X-Ray Observatory. Here we report on the optical counterpart for the ULX in NGC 5204, which has average X-ray luminosity of similar to3x10(39) ergs s(-1) 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 far-ultraviolet (FUV) spectrum of this object show that it is a B0 Ib supergiant star with peculiarities, including the N v lambda1240 emission line, which 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 similar to10 days for different black hole masses, which can be tested by future observations.