High-resolution imaging of the HeII λ4686 emission line nebula associated with the ultraluminous X-ray source in Holmberg II

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 = 08(h) 19(m) 28(s).98, delta = + 70degrees42'19.3 (J2000) with an uncertainty of 0.6 arcsec. We identify a bright, point-like optical counterpart centred in the nebula with the X-ray source. The optical magnitude and colour 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 disc. The nebular He II luminosity is 2.7 x 10(36) erg s(-1). The morphology of the He II, Hbeta and [O I] emission is consistent with being due to X-ray photoionization and is inconsistent with narrow beaming of the X-ray emission. A spectral model consisting of a multicolour disc 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 x 10(39) erg s(-1) if the extrapolation of the X-ray spectrum between 54 and 300 eV is accurate. A compact object mass of at least 25 to 40 M. would be required to avoid violating the Eddington limit.