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

TL;DR

This study analyzes UV and X-ray data of Holmberg II's ultraluminous X-ray source, suggesting its UV/optical emission is primarily from an X-ray irradiated disk rather than stellar wind or normal stellar spectra.

Contribution

It provides the first detailed UV/optical spectral analysis of Holmberg II X-1, favoring an irradiated disk model over stellar models for the source's emission.

Findings

Irradiated disk model fits UV/optical data well

Normal metallicity stellar spectra are inconsistent with observations

Low metallicity stellar spectra are marginally acceptable

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.