Radio investigation of Ultra-Luminous X-ray Sources in the SKA Era
  A puzzling class of exotic objects, which have been known about for more than
30 years, is reaching a new era of understanding. We have discovered hundreds
of Ultra Luminous X-ray sources (ULXs) - non-nuclear sources with X-ray
luminosity in excess of the Eddington luminosity for "normal" size stellar
Black Holes (BH) - and we are making progresses towards understanding their
emission mechanisms. The current explanations imply either a peculiar state of
accretion onto a stellar size BH or the presence of an intermediate mass BH,
the long-sought link between stellar and supermassive BHs. Both models might
co-exist and therefore studying this class of object will give insight into the
realm of accretion in a variety of environments and at the same time find
look-alikes of the primordial seed BHs that are thought to be at the origin of
todays supermassive BHs at the centre of galaxies. The radio band has been
exploited only scantily due to the relative faint fluxes of the sources, but we
know a number of interesting sources exhibiting both extended emission (like
bubbles and possibly jets) and cores, as well as observed transient behaviour.
The new eras of the SKA will lead us to a major improvement of our insight of
the extreme accretion within ULXs. We will both investigate in detail known
sources and research new and fainter ones. When we have reached a thorough
understanding of radio emission in ULX we could also use the SKA as a discovery
instrument for new ULX candidates. The new array will give an enormous space to
discovery: sources like the ones currently known will be detected in a snapshot
up to 50 Mpc instead of at 5 Mpc with long, pointed observations.
