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 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 ⊙, 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 Holmberg II X-1 may be dominated by X-ray irradiated disk emission.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2012
- DOI:
- 10.1088/0004-637X/750/2/110
- arXiv:
- arXiv:1203.1086
- Bibcode:
- 2012ApJ...750..110T
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- galaxies: individual: Holmberg II;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 9 pages, 1 table, 6 figures, accepted by ApJ