  In this letter, we comment on the robustness of putative cool (kT ~ 0.2 keV)
accretion disc components in the X-ray spectra of the most luminous (L ~ 10^40
erg/s) ultra-luminous X-ray sources (ULXs) in nearby normal galaxies. When
compared to stellar-mass black holes, the low disc temperatures observed in
some ULXs may imply intermediate-mass black hole primaries. Recent work has
claimed that such soft excesses are unlikely to be actual disc components,
based on the lack of variability in these components, and in the overall source
flux. Other work has proposed that alternative phenomenological models, and
complex Comptonisation models, rule-out cool disc components in ULX spectra. An
inspection of the literature on Galactic stellar-mass black holes and black
hole candidates demonstrates that the flux behaviours seen in specific ULXs are
consistent with phenomena observed in well-known Galactic X-ray binaries.
Applying Comptonisation models to simulated disc blackbody plus power-law
spectra shows that at the sensitivity achieved in even the best ULX spectra,
Comptonisation fits are highly model-dependent, and do not yield meaningful
constraints on the accretion flow. In contrast, the need for a soft, thermal
component does not appear to be model-dependent. As we have previously noted,
soft thermal components in ULX spectra may not represent accretion discs, but
present alternatives to this interpretation are not robust.
