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J. Liu, J.N. Bregman, P. Seitzer (University of Michigan)
The ultraluminous X-ray sources (also, IXO) are the most luminous non-nuclear sources in galaxies (1039-1040 erg/s) and may be intermediate mass black holes (103-104 Msolar). To search for their optical counterparts, we combined high-resolution X-ray imaging from Chandra with WFPC images from HST. We have identified the optical counterparts to IXOs in M81 and M51 and improve the previous optical identification in NGC 2403. The counterparts are blue objects and in M81, the colors and magnitude are consistent with a single O star, which we presume to be the secondary in a close binary system. These results support the view that these are short-lived systems involving massive stars. Now that the secondaries are being identified, we need only the period of the binary to determine whether these are indeed intermediate mass black holes.