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We present the first Spitzer Infrared Spectrograph (IRS) observations of the[O IV] 25.89 $\mu$m emission line detected from the ultraluminous X-ray source(ULX) in Holmberg II. This line is a well established signature of highionization, usually associated with AGN. Its detection suggests that the ULXhas a strong impact on the surrounding gas. A Spitzer high resolution spectralmap shows that the [O IV] is coincident with the X-ray position of the ULX. Theratios of the [O IV] to lower ionization lines are similar to those observed inAGN, suggesting that a strong UV and X-ray source is responsible for thephotoionization. The best XMM-Newton data is used to model the X-ray band whichis then extrapolated into the UV. We perform infrared and ultravioletphotometry, and use previously published optical and radio data to constructthe full SED for the ULX and its companion. The preferred model to describe theSED includes an accretion disk which dominates the soft X-rays but contributeslittle at UV and optical wavelengths. The optical counterpart is consistentwith a B supergiant as previously suggested in other studies. The bolometricluminosity of the ULX suggests the presence of an intermediate-mass black holewith mass $>$85 M$_\odot$ for sub-Eddington accretion or, alternatively, astellar-mass black hole that is accreting at super-Eddington rates. In afollow-up second paper we perform detailed photoionization modeling of theinfrared lines in order to constrain the bolometric luminosity of the ULX.

The First Detection of [O IV] from an Ultraluminous X-ray Source with Spitzer I. Observational Results for Holmberg II ULX