Dynamics of massive stellar black holes in young star clusters and the
  displacement of ultra-luminous X-ray sources
  In low-metallicity environments, massive stars might avoid supernova
explosion and directly collapse, forming massive (~25-80 solar masses) stellar
black holes (MSBHs), at the end of their life. MSBHs, when hosted in young
massive clusters, are expected to form binaries and to strongly interact with
stars, mainly via three-body encounters. We simulate various realizations of
young star clusters hosting MSBHs in hard binaries with massive stars. We show
that a large fraction (~44 per cent) of MSBH binaries are ejected on a short
timescale (<=10 Myr). The offset of the ejected MSBHs with respect to the
parent cluster is consistent with observations of X-ray binaries and
ultra-luminous X-ray sources. Furthermore, three-body encounters change the
properties of MSBH binaries: the semi-major axis changes by <=50 per cent and
the eccentricity of the system generally increases. We shortly discuss the
implications of our simulations on the formation of high-mass X-ray binaries
hosting MSBHs.
