Logo (Main page)

The Ultraluminous X-ray Source HoII X-1:
Flight through the Interstellar Medium

Russian version

    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.
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.