  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 100-1000 solar masses, which have led
to the idea that this might 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
stellar-mass BHs accreting at an unusual super-Eddington regime. In this paper
we briefly describe the model SLIMULX that can be used in XSPEC for the fit of
thermal spectra of slim discs around stellar mass black holes in the
super-Eddington regime. This model consistently takes all relativistic effects
into account. We present the obtained results from the fit of the X-ray spectra
from NGC 5408 X-1.
