The Nature of the UV/optical Emission of the Ultraluminous X-Ray Source in Holmberg II
Abstract
We report on UV and X-ray spectroscopy and broad-band optical observations of the ultraluminous X-ray source in Holmberg II. 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 chi2/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 Holmberg II X-1 may be dominated by X-ray irradiated disk emission.
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