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 time scales 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 _ { \nu } \propto \nu ^ { \alpha } with \alpha in the range 1.0 to 2.0 and the optically emitting region has a size on the order of 10 ^ { 12 } cm .
Exceptions are NGC 2403 X-1 and M83 IXO 82 , which show optical spectra consistent with direct emission from a standard thin disk , M101 ULX-1 and M81 ULS1 , which have X-ray to optical flux ratios more similar to high-mass X-ray binaries , and IC 342 X-1 , in which the optical light may be dominated by the companion star .
Inconsistent extinction between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae suggests that they may be unrelated .