  Ultra-Luminous X-ray sources (ULXs) are accreting black holes for which their
X-ray properties have been seen to be different to the case of stellar-mass
black hole binaries. For most of the cases their intrinsic energy spectra are
well described by a cold accretion disc (thermal) plus a curved high-energy
emission components. The mass of the black hole (BH) derived from the thermal
disc component is usually in the range of 100-10^5 solar masses, which have led
to the idea that this can represent strong evidence of the Intermediate Mass
Black Holes (IMBH), proposed to exist by theoretical studies but with no firm
detection (as a class) so far. Recent theoretical and observational
developments are leading towards the idea that these sources are instead
compact objects accreting at an unusual super-Eddington regime instead. On the
other hand, gravitational waves have been seen to be a useful tool for finding
(some of these) IMBHs. We give a brief overview about the recent advent of the
discovery of gravitational waves and their relationship with these so far
elusive IMBHs.
