The First Detection Of [O IV] From An Ultraluminous X-ray Source With Spitzer: Evidence Of High Unbeamed Luminosity In Holmberg II ULX
Abstract
We present the first Spitzer Infrared Spectrograph observations of the [O IV] 25.89 μ m emission line detected from the ULX in Holmberg II. This line is a well established signature of high excitation, usually associated with AGN. Its detection suggests that the ULX has a strong impact on the surrounding gas. A Spitzer high resolution spectral map shows that the [O IV] is coincident with the X-ray position of the Holmberg II ULX. We find that the luminosity and the morphology of the line emission is consistent with photoionization by the soft X-ray and far ultraviolet radiation from the accretion disk of the binary system and inconsistent with narrow beaming. We show that the emission nebula is matter bounded both in the line of sight direction and to the east, and probably radiation bounded to the west. Evidence for a massive black hole in this ULX is mounting. Detailed photoionization models favor an intermediate mass black hole of at least 85 M⊙ as the ionization source for the [O IV] emission. We find that the spectral type of the companion star strongly affects the expected strength of the [O IV] emission. This finding could explain the origin of [O IV] in some starburst galaxies containing black hole binaries.
- Publication:
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American Astronomical Society Meeting Abstracts #213
- Pub Date:
- January 2009
- Bibcode:
- 2009AAS...21330804B