The nature of ultraluminous X-ray sources (ULXs) remains a subject of
controversy and debates for past two decades.
Some researchers believe that the huge X-ray luminosity
(1039-1041 erg/s),
exceeding the Eddington limit, is caused by an
accretion of matter onto hypothetical intermediate-mass black holes
(in the range from 100 to several thousands of solar
masses). An alternative view, actively suggested by SAO RAS researchers
also
(interview
by Sergei Fabrika) explains the
ULX phenomenon by accretion onto the "normal" stellar-mass black holes,
but their radiation is anisotropic, so that the
maximum is observed along the a accretion disk axis. To make a choice
between different hypotheses, it is important to
study the impact of the ULX on the ambient medium.
Ionized gas motions in the "Foot Nebulae" related with the ULX in the
nearby galaxy Holmberg II were studied in detail at
the 6-m SAO RAS telescope with the
SCORPIO and
SCORPIO-2
instruments working in the scanning Fabry-Perot
interferometer mode. In addition to the expansion of an ionized gas
superbubble around the young star cluster in the
nebulae, a characteristic "arc" in the distribution of line-of-sight
velocities was found. The velocities in this arc are shifted
by 10-15 km/s to the observer. This features is observed simultaneously
in three emission lines of the ionized gas. It can be
explained by the bow shock caused by the fast flight of the ULX through
the interstellar medium and by the influence on its
hot wind on the nebula. Selection of the parameters of the bow shock
formation model provided the best agreement with
observations under the assumption that the ULX moves from the the central
part of the nearby star cluster at a speed of
about 70 km/s (see the Figure). This hypothesis is fully consistent with
the estimates of the age of the cluster and the
distance to it. If the proposed interpretation is correct, then the
observations at the 6-m telescope for the first time ever
allowed one to directly detect how an ultraluminous X-ray source escapes
from the parent star cluster. Recently, the
ejection of ULXs from clusters was found from the analysis of the
location of X-ray sources beyond the boundaries of
clusters in several galaxies (Poutanen Ju., Fabrika S. et al.,
2013, MNRAS,
432, 506, see
news on the SAO
page). Such ejection can occur due to the close approach of stars
with the the binary system - a precursor of the ULX, in early stages of
cluster evolution. Thus, new observations support the idea that the
ULXs are accreting black holes of stellar masses.
Published:
Egorov O.V., Lozinskaya T.A., Moiseev A.V., 2017, MNRAS, 467, L1;
arXiv:1611.09684 [astro-ph.GA]
Contact -
Oleg Egorov,
Tatiana Lozinskaya,
Alexei Moiseev
Fig.1.
Observations of "Foot Nebulae" at the 6-m SAO RAS telescope. Line-of-sight
velocity fields of the ionized hydrogen
(left), sulfur (middle), and oxygen (right) are shown. Contours mark the
brightness distribution in the ionized gas emission
line, a red star denotes the position of the ULC, a grey dashed ellipse
shows the borders of a young cluster. A red curve
shows the modeled shape of a bow shock that might be induced by the ULX
motion from the cluster center in the direction shown with an arrow.
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