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J. Liu, J. Bregman (University of Michigan)
Ultra-luminous X-Ray sources are extra-nuclear point sources in external galaxies with LX=1039--1041 erg/s and are among the most poorly understood X-ray sources. To help understand their nature, we are trying to identify their optical counterparts by combining images from the Hubble Space Telescope and the Chandra Observatory plus the STIS FUV spectrum. Here we report upon the optical counterpart for the ULX in NGC 5204, which has an average X-ray luminosity of ~3\times1039 erg/s and has varied by a factor of 50% over the last 10 years. A unique optical counterpart to this ULX is found by carefully comparing the Chandra ACIS images and HST WFPC2 and ACS/HRC images. The spectral energy distribution and the FUV spectrum of this object show that it is a B0 Ib supergiant star with peculiarities, including the \lambda1240 N V emission line that is uncommon in B stellar spectra but has been predicted for X-ray illuminated accretion disks and seen in some X-ray binaries. A study of its FUV spectrum leads to a binary model for this ULX in which the B0 Ib supergiant is overflowing its Roche Lobe and accreting onto the compact primary, probably a black hole. This model predicts an orbital period of ~10 days for different black hole mass, which can be tested by future observations.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.