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Date and Time of the Query: 2019-05-26 T07:35:40 PDT
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For refcode 2012ApJ...750..110T:
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2012ApJ...750..110T The Nature of the UV/Optical Emission of the Ultraluminous X-Ray Source in Holmberg II Tao, Lian; Kaaret, Philip; Feng, Hua; Grise, Fabien Abstract. We report on UV and X-ray spectroscopy and broadband 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_&sun;_, 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 chi^2^/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. Key words: accretion, accretion disks, black hole physics, galaxies: individual: Holmberg II, X-rays: binaries
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