High Resolution Imaging of the He ii λ 4686 Emission Line Nebula Associated with the Ultraluminous X - Ray Source in Holmberg II

We present Hubble Space Telescope images of the Heiii region surrounding the bright X-ray source in the dwarf irregular galaxy Holmberg II. Using Chandra, we find a position for the X-ray source of α = 08h 19m 28.98s, δ = +70 • 42 ′ 19. ′′ 3 (J2000) with an uncertainty of 0.6 ′′. We identify a bright, point-like optical counterpart centered in the nebula with the X-ray source. The optical magnitude and color of the counterpart are consistent with a star with spectral type between O4V and B3 Ib at a distance of 3.05 Mpc or reprocessed emission from an X-ray illuminated accretion disk. The nebular Heii luminosity is 2.7 × 10 36 erg s −1. The morphology of the Heii, Hβ, and [Oi] emission are consistent with being due to X-ray photoionization and are inconsistent with narrow beaming of the X-ray emission. A spectral model consisting of a multicolor disk blackbody with inverse-Compton emission from a hot corona gives a good fit to X-ray spectra obtained with XMM-Newton. Using the fitted X-ray spectrum, we calculate the relation between the Heii and X-ray luminosity and find that the Heii flux implies a lower bound on the X-ray luminosity in the range 4 to 6 × 10 39 erg s −1 if the extrapolation of the X-ray spectrum between 54 eV and 300 eV is accurate. A compact object mass of at least 25 to 40M ⊙ would be required to avoid violating the Eddington limit.