In this letter , we comment on the robustness of putative cool ( kT \simeq 0.2 keV ) accretion disc components in the X-ray spectra of the most luminous ( L _ { X } \simeq 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 .