2010MNRAS.404...42B

We examined the variability of three ultraluminous X-ray sources (ULXs) in the 2003, 110ks XMM-Newton observation of NGC253. 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.