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.
Contact - S.Fabrika
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
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