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| - Spectral variability of ultraluminous X-ray sources
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| - We study spectral variability of 11 ultraluminous X-ray sources (ULX) using archived XMM-Newton and Chandra observations. We use three models to describe the observed spectra: a power law, a multicolour disc (MCD) and a combination of these two models. We find that seven ULXs show a correlation between the luminosity LX and the photon index Γ. Furthermore, four out of these seven ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an LX-Γ anticorrelation. The spectra of four ULXs in the sample can be adequately fitted with a MCD model. We compare these sources to known black hole binaries (BHB) and find that they follow similar paths in their luminosity-temperature diagrams. Finally, we show that the ‘soft excess’ reported for many of these ULXs at ∼0.2 keV seems to roughly follow a trend Lsoft∝T−3.5 when modelled with a power law plus a ‘cool’ MCD model. This is contrary to the L∝T4 relation that is expected from theory and what is seen for many accreting BHBs. The observed trend could instead arise from disc emission beamed by an outflowing wind around a ∼10 M⊙ black hole.
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