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

By P. Kaaret, M. Ward, A. Zezas. 2004

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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.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/c29284304670d6c09e68140e09f0481a2e9e9e5a

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In Astronomy and Astrophysics

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Citations in the corpus

• Ultraluminous X-ray sources in the Chandra and XMM-Newton era (2011, background): “Conversely, the presence of photoionized bubbles around several ULXs (for example Holmberg II X-1: Pakull & Mirioni 2002; Kaaret et al. 2004; Lehmann et al. 2005; NGC 5408 X-1: Kaaret & Corbel 2009) requires quasiisotropic X-ray emission with luminosities≈ 1040 erg s−1.”
• Populations of X-Ray Sources in Galaxies (2006, background): “F or p er so na l u se o nl y. sources experience formation kicks, displacing them from their parent star cluster (Kaaret et al. 2004), as observed in the Milky Way.”
• THE ORIGIN AND OPTICAL DEPTH OF IONIZING RADIATION IN THE “GREEN PEA” GALAXIES (2013, background): “Kaaret et al. (2004) report the detection of strong He ii λ4686 emission, with a luminosity of 2.7×1036 ergs s−1 , around a ULX nebula in Holmberg II; the galaxy has a metallicity near 0.2 Z⊙(Moustakas et al. 2010).”
• Supercritically accreting stellar mass black holes as ultraluminous X-ray sources (2006, background): “Important clues on the nature of ULXs come from the presence of extended photoionized nebulae around them (Wang 2002; Pakull & Mirioni 2003; Kaaret et al. 2004).”