Ultraluminous X-ray sources (ULXs) are located in external galaxies, their X-ray luminosities (10³⁹ - 10⁴¹ erg/s) are thousand times greater than those of brightest black holes in the Galaxy. ULXs may be supercritical accretion disks around black holes of stellar masses (~10 Solar masses) like that in SS433, or they may be a hypothetical intermediate-mass black holes (hundreds - thousands Solar masses) with standard accretion disks.
    We have carried out observations of stellar clusters related to the ULXs, in South sky there were observations with the VLT (VIMOS, MOS) of clusters in the Antennae galaxies (Fig. 1 and 2) and in NGC3256, in Northern sky with the BTA (Scorpio) of clusters in NGC4485/4490 galaxies. We have discovered that all the ULXs are located nearby to (inside about 200 pc) very young star clusters (Fig. 3), all the clusters are younger than 5 Myrs. A probability of occasional associations of the ULXs with the clusters is very little (0.0000001), it is even less for young clusters.
    We conclude:
1) The ULX progenitors are close binary systems with their star masses not less than 50-100 Solar masses. They are black holes of "stellar masses" with a supercritical accretion.
2) The ULX progenitors were ejected from the star clusters. The ejections because of the Supernova explosion can not explain these data, there is not a time both for stellar evolution and for the transportation from the cluster to outside. The ejection occurs due to 3- and 4-body encounters in the cluster centers at the very beginning stages of cluster evolution.
3) This confirmes the new ideas of stellar clusters formation. This mechanism explains the well-known problem in our Galaxy - the problem of massive run-away stars.
Published:
J.Poutanen, S.Fabrika, A.Valeev, O.Sholukhova, J.Greiner, arXiv1210.1210