2005ApJ...620L..31K

We have identified the optical counterpart of the ultraluminous X-ray source M101 ULX-1 (CXOKM101 J140332.37+542102), by comparing Hubble Space Telescope Advanced Camera for Surveys images with Chandra ACIS-S images. The optical counterpart has V=23.75 and colors consistent with those for a mid-B supergiant. Archival Wide Field Planetary Camera 2 observations show that the source brightness is constant to within ∼0.1 mag. The physical association of this source with the ULX is confirmed by Gemini Multi-Object Spectrograph observations that show spatially unresolved He II λ4686 and He I λ5876 emission. These results suggest that M101 ULX-1 is a high-mass X-ray binary, but deep spectroscopic monitoring observations are needed to determine the detailed properties of this system.