  Despite two decades of studies, it is still not clear whether ULX spectral
transitions are due to stochastic variability in the wind or variations in the
accretion rate or in the source geometry. The compact object is also unknown
for most ULXs. In order to place constraints onto such scenarios and on the
structure of the accretion disc, we studied the temporal evolution of the
spectral components of the variable source NGC 55 ULX-1. Using recent and
archival data obtained with the XMM-Newton satellite, we modelled the spectra
with two blackbody components which we interpret as thermal emission from the
inner accretion flow and the regions around or beyond the spherization radius.
The luminosity-temperature (L-T) relation of each spectral component agrees
with the L proportional T^4 relationship expected from a thin disc model, which
suggests that the accretion rate is close to the Eddington limit. However,
there are some small deviations at the highest luminosities, possibly due to an
expansion of the disc and a contribution from the wind at higher accretion
rates. Assuming that such deviations are due to the crossing of the Eddington
or supercritical accretion rate, we estimate a compact object mass of 6-14
Msun, favouring a stellar-mass black hole as the accretor.
