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Paper: Ultraluminous X-ray Sources
Volume: 510, Stars: From Collapse to Collapse
Page: 395
Authors: Fabrika, S.
Abstract: The origin of Ultraluminous X-ray sources (ULXs) in external galaxies whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times is mysterious. Here we report that all nearby persistent ULXs ever spectroscopically observed have the same optical spectra similar to that of SS 433, the only known supercritical accretor in our Galaxy. The spectra are apparently of WNL type (late nitrogen Wolf-Rayet stars) or LBV (luminous blue variables) in their hot state, which are very scarce stellar objects. We find that the spectra do not originate from WNL/LBV type donors and not in heated accretion disks, but from very hot winds from the accretion disks, which have similar physical conditions as the stellar winds from these stars. Our results suggest that bona-fide ULXs must constitute a homogeneous class of objects, which most likely have supercritical accretion disks.
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