Ultraluminous X-Ray Objects are off-nucleus point sources 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 an optical counterpart for ULX NGC 3031 X-11 , which has average X-ray luminosity of \sim 2 \times 10 ^ { 39 } erg/s and has varied by a factor of 40 % over the last 20 years .
We find a unique optical counterpart with the magnitude and color of an O8V star and we identify this as the secondary in a binary system .
The primary is believed to be a black hole of approximately 18 M _ { \odot } , based on analyses of ACIS and ASCA spectra .
This binary system probably is powered by mass transfer from the O8V star onto the black hole , with the Roche Lobe equal to the stellar radius .
This model predicts an orbital period of \sim 1.8 days , which can be tested by future observations .