The nature of the UV/optical emission of the ultraluminous X-ray source in Holmberg II

We report on UV and X-ray spectroscopy and broadband optical observations of the ultraluminous X-ray source in HolmbergII. Fitting various stellar spectral models to the combined, non-simultaneous data set, we find that normal metallicity stellar spectra are ruled out by the data, while low-metallicity, Z = 0.1 Z_̇, late O-star spectra provide marginally acceptable fits, if we allow for the fact that X-ray ionization from the compact object may reduce or eliminate UV absorption/emission lines from the stellar wind. By contrast, an irradiated disk model fits both UV and optical data with χ²/dof = 175.9/178, and matches the nebular extinction with a reddening of E(B - V) = 0.05^+0.05 _{- 0.04}. These results suggest that the UV/optical flux of HolmbergII X-1 may be dominated by X-ray irradiated disk emission.