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arXiv · 1203.1086

The Nature of the UV/optical Emission of the Ultraluminous X-Ray Source in Holmberg II

Lian Tao·Philip Kaaret·Hua Feng·Fabien Grisé
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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_{\odot}, 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.

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Lian Tao, Philip Kaaret, Hua Feng, Fabien Grisé. 2012-03-06. The Nature of the UV/optical Emission of the Ultraluminous X-Ray Source in Holmberg II. https://doi.org/10.1088/0004-637x/750/2/110

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