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

Kaaret, P. and Ward, M. J. and Zezas, A. (2004) 'High-resolution imaging of the He II λ4686 emission line nebula associated with the ultraluminous X-ray source in Holmberg II.', Monthly notices of the Royal Astronomical Society., 351 (3). L83-L88.


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 (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 × 1036 erg s-1. The morphology of the He ii, Hβ 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 × 1039 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.

Item Type:Article
Keywords:Black hole physics, Individual, Holmberg II, Starburst, Stellar content, X-rays.
Full text:(NA) Not Applicable
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Date accepted:No date available
Date deposited:No date available
Date of first online publication:July 2004
Date first made open access:No date available

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