The Optical Counterpart of M101 ULX-1

We have identified the optical counterpart of the Ultra-Luminous X-ray source Ml0l ULX-1 (CX- OKM101 J140332.74+542102), by comparing HST ACS images with Chandra ACIS-S images. The optical counterpart has V= 23.75 and colours consistent with those for a mid-B supergiant. Archival WFPC2 observations show that the source brightness is constant to within approximately 0.1 mag. The physical association of this source with the ULX is confirmed by Gemini GMOS spectroscopic observations which show spatially unresolved He II lambda4686 and He I lambda5876 emission. These results suggest that M10l ULX-1 is a HMXB but deep spectroscopic monitoring observations are needed to determine the detailed properties of this system.