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تسجيل مستخدم جديدHigh Resolution Imaging of the HeII lambda 4686 Emission Line Nebula Associated with the Ultraluminous X-Ray Source in Holmberg II
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We present Hubble Space Telescope images of the He{sc iii} region surrounding the bright X-ray source in the dwarf irregular galaxy Holmberg II. Using {it Chandra}, we find a position for the X-ray source of $alpha=$ 08h 19m 28.98s, $delta=$ +70arcdeg 42arcmin 19.arcsec3 (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 He{sc ii} luminosity is $2.7 times 10^{36} rm erg s^{-1}$. The morphology of the He{sc ii}, H$beta$, and [O{sc 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{sc ii} and X-ray luminosity and find that the He{sc ii} flux implies a lower bound on the X-ray luminosity in the range 4 to $6 times 10^{39} rm erg s^{-1}$ 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.
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