Spectral variability of ultraluminous X-ray sources

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 L(X) and the photon index Gamma. Furthermore, four out of these seven ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an L(X)-Gamma 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 similar to 0.2 keV seems to roughly follow a trend L(soft) proportional to T(-3.5) when modelled with a power law plus a 'cool' MCD model. This is contrary to the L proportional to T(4) 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 similar to 10M(circle dot) black hole.