Compact optical counterparts of ultraluminous x-ray sources

Using archival Hubble Space Telescope (HST) imaging data, we report the multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that have a unique compact optical counterpart. Both magnitude and color variation are detected at timescales of days to years. The optical color, variability, and X-ray to optical flux ratio indicate that the optical emission of most ULXs is dominated by X-ray reprocessing on the disk, similar to that of low-mass X-ray binaries. For most sources, the optical spectrum is a power law, F _νν^α with α in the range 1.0-2.0 and the optically emitting region has a size on the order of 10¹²cm. Exceptions are NGC2403 X-1 and M83 IXO 82, which show optical spectra consistent with direct emission from a standard thin disk, M101ULX-1 and M81ULS1, which have X-ray to optical flux ratios more similar to high-mass X-ray binaries, and IC342 X-1, in which the optical light may be dominated by the companion star. Inconsistent extinction between the optical counterpart of NGC5204 X-1 and the nearby optical nebulae suggests that they may be unrelated.