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Ultraluminous X-ray sources
as supercritical accretion disks

Russian version

    Using optical spectroscopy a nature of ultraluminous X-ray sources in external galaxies whose X-ray luminosities exceed the luminosities of the brightest black holes in our Galaxy by thousands of times was revealed.
    The ultraluminous X-ray sources (ULX) were first discovered in end of 1990s. Their nature is yet unknown. There are two interpretations of ULX: either they are intermediate-mass black holes with standard accretion disks or they are supercritical (super-Eddington) disks with stellar-mass black holes whose X-ray radiation is collimated in the funnel of the supercritical disk. These objects are very faint in visible light, about 21 - 23 magnitude or fainter. With optical spectroscopy we have resolved the nature of these objects. We obtained several spectra of the nearest ULXs: four with the Subaru telescope, two with the 6-m BTA telescope, and three VLT spectra were reduced by us using archive data. These are all the ULXs whose spectra have even been obtained nowadays. As it turned out, all the 9 objects have very rare spectral type of WNLh. Such a spectrum is observed in LBV stars (luminous blue variables) during their hot state. When LBV star expands, its photosphere cools down to about 10000-15000 degrees, and when the star shrinks, its temperature increases up to 35000-40000 degrees (hot state). The same type of the spectrum is observed in SS433, the only known supercritical accretor in the Galaxy. Both in SS433 and in LBVs we see very hot and dense winds. Because all the known ULXs show the same rare type of optical spectrum, we conclude that they are a homogeneous class of objects, and their nature is supercritical accretion disks in close binaries with stellar-mass black holes.
S.Fabrika, Y.Ueda, A.Vinokurov, O.Sholukhova, M.Shidatsu, Nature Physics, 11, 551, 2015

Contact - S.Fabrika

Fig.1. Optical spectra of ULX taken with Subaru telescope. blue range on the left, red range on the right. From top to bottom: the ULXs in galaxies Holmberg II, Holmberg IX, NGC4559, and NGC5204. The brightest lines are those of ionised helium He II, and the Balmer H-alpha and H-beta lines. The broad He I line of neutral helium is also detected. Narrow nebular emission lines are oversubstacted in the two bottom spectra