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We examined the variability of three ultraluminous X‐ray sources (ULXs) in the 2003, 110 ks  XMM–Newton  observation of NGC 253. Remarkably, we discovered ULX1 to be three times more variable than ULX2 in the 0.3–10 keV band, even though ULX2 is brighter. Indeed, ULX1 exhibits a power density spectrum that is consistent with the canonical high state or very high/steep power‐law states, but not the canonical low state. The 0.3–10 keV emission of ULX1 is predominantly non‐thermal, and may be related to the very high state. We also fitted the ULX spectra with disc blackbody, slim disc and convolution Comptonization ( simpl⊗diskbb ) models. The brightest ULX spectra are usually described by two emission components (disc blackbody + Comptonized component); however, the  simpl  model results in a single emission component, and may help determine whether the well‐known soft excess is a feature of ULX spectra or an artefact of the two‐component model. The  simpl  models were rejected for ULX3 (and also for the black hole + Wolf–Rayet binary IC10 X‐1); hence, we infer that the observed soft‐excesses are genuine features of ULX emission spectra. We use an extended corona scenario to explain the soft excess seen in all the highest quality ULX spectra, and provide a mechanism for stellar mass black holes to exhibit super‐Eddington luminosities while remaining locally sub‐Eddington.

In‐depth studies of the NGC 253 ULXs with XMM–Newton : remarkable variability in ULX1, and evidence for extended coronae