High-resolution imaging of the He II λ4686 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 � = 08h 19m 28.98s, � = +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 ergs −1 . The morphology of the Heii, H�, 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 ergs −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.