SPITZER IRAC OBSERVATIONS OF IR EXCESS IN HOLMBERG IX X-1: A CIRCUMBINARY DISK OR A VARIABLE JET?

We present Spitzer Infrared Array Camera photometric observations of the ultraluminous X-ray source (ULX, X-1) in Holmberg IX. We construct a spectral energy distribution (SED) for Holmberg IX X-1 based on published optical, UV, and X-ray data combined with the IR data from this analysis. We modeled the X-ray and optical data with disk and stellar models; however, we find a clear IR excess in the ULX SED that cannot be explained by fits or extrapolations of any of these models. Instead, further analysis suggests that the IR excess results from dust emission, possibly a circumbinary disk, or a variable jet.