Ultraluminous X-ray sources: a deeper insight into their spectral evolution

We select a sample of nearby ultraluminous X-ray sources with long XMM-Newton observations and analyse all the available XMM-Newton data using both X-ray spectral fitting techniques and hardness-intensity diagrams. The sample includes IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1, NGC 1313 X-1, NGC 1313 X-2 and NGC 253 X-1. We found that, although a common reference model can be used to describe the X-ray spectra, the sources show different spectral evolutions, phenomenologically described in terms of variations in the properties of a soft component and a high-energy tail. Variations at low energies are accounted for (mostly) by changes in the normalization of the soft component and/or in the column density to the source, while variations in the high-energy tail by changes in the parameters of an optically thick corona. This spectral variability is rather well characterized on a colour-colour and hardness-intensity diagram in terms of suitably defined hardness ratios. We suggest the existence of a variability pattern on the hardness-intensity diagram and we interpret it in terms of the switch between a near-Eddington and a super-Eddington accretion regime. The transition between the two regimes seems to be driven mostly by changes in the contribution of the soft component, which can be explained in terms of the increasing importance of wind emission. The analysis is complemented by an investigation of the short-term time variability of all the sources. In general, no clear correlation between the spectral and temporal properties is found.