We present Hubble Space Telescope images of the He iii 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 \alpha = 08h 19m 28.98s , \delta = +70 ^ { \circ } 42′19.″3 ( J2000 ) with an uncertainty of 0.6 ^ { \prime \prime } .
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 He ii luminosity is 2.7 \times 10 ^ { 36 } erg s ^ { -1 } .
The morphology of the He ii , H \beta , and [ O i ] 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 He ii and X-ray luminosity and find that the He ii flux implies a lower bound on the X-ray luminosity in the range 4 to 6 \times 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 40 M _ { \odot } would be required to avoid violating the Eddington limit .