Optical Counterparts of Ultraluminous X-Ray Sources

Abstract

Despite much observational and theoretical effort little is presently known about the nature of the luminous non-nuclear X-ray sources which appear to largely surpass the Eddington limit of a few solar masses. Here we present first results of our OHP/ESO/CFHT optical survey of the environments of variable ultraluminous X-ray sources (ULX) in nearby galaxies. At the position of several ULX we find emission nebulae of a few hundred parsecs diameter, and which often show both low and high ionisation emission lines. The gas must therefore be either photoionized by hard XUV continua, or be shock-ionized in the expanding bubbles. The nebulae have kinematic ages of some million years and appear to be directly linked to the highly energetic formation process of the compact ULX or being inflated by ongoing stellar wind/jet activity. The discovery of intense HeII lambda4686 nebular recombination radiation together with comparatively strong [OI] lambda6300 emission around the variable ULX in dwarf galaxy Holmberg II has allowed us to show that the interstellar medium acually 'sees' and reprocesses part of the 10^40 erg/s measured at X-ray wavelengths, if we assume isotropic emission. Strong beaming into our line of sight which has been advocated to avoid such high luminosities can thus be excluded, at least for this source.

arXiv comment8 pages, 8 figures, to appear in the proceedings of the symposium 'New Visions of the X-ray Universe in the XMM-Newton and Chandra Era', 26-30 November 2001, ESTEC, The Netherlands

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• Optical Counterparts of Ultra-Luminous X-ray Sources identified from Archival Hubble Space Telescope/WFPC2
• (No) dynamical constraints on the mass of the black hole in two ULXs
• The Physical Parameters of the Micro-quasar S26 in the Sculptor Group Galaxy NGC 7793
• The ultraluminous X-ray source bubble in NGC 5585
• An Off-Nucleus Nonstellar Black Hole in the Seyfert Galaxy NGC 5252
• Models of Ultra-Luminous X-ray transient sources
• The ultraluminous X-ray source HoII X-1: kinematic evidence of its escape from the cluster
• X-ray Spectral Signatures of the Photon Bubble Model for Ultraluminous X-ray Sources
• Toward a Clean Sample of Ultra-Luminous X-ray Sources
• Dynamical mass constraints on the ultraluminous X-ray Source NGC 1313 X-2
• Spectral State Transitions of the Ultraluminous X-ray Source IC 342 X-1
• The Binary Black Hole Merger Rate from Ultraluminous X-ray Source Progenitors
• VLT/FORS2 observations of four high-luminosity ULX candidates
• Variability in Ultra-luminous X-ray Sources
• Optical study of the hyper-luminous X-ray source 2XMM J011942.7+032421
• Spitzer IRAC observations of IR excess in Holmberg IX X-1: A circumbinary disk or a variable jet?
• Multiwavelength study of the bright X-ray source population in the interacting galaxies NGC5774/NGC5775
• The High Energy X-ray Probe (HEX-P): Studying Extreme Accretion with Ultraluminous X-ray Sources
• Black Hole Mass Limits for Optically Dark X-ray Bright Sources in Elliptical Galaxies
• The FRII Broad Line Seyfert 1 Galaxy: PKSJ 1037-2705
• The Ultraluminous X-ray Sources NGC 1313 X-1 and X-2
• X-raying the galaxy pair Arp 41: no collision in NGC 1232 and three ultraluminous sources in NGC 1232A