Poster abstract details

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 spectra: a powerlaw, a multi-colour disc (MCD) and a combination of these two models. We find that 7 ULXs show a correlation between the luminosity $L_x$ and the photon index and 4 out of these 7 ULXs also show spectral pivoting in the observed energy band. The spectra of 4 ULXs in the sample can be fitted with a MCD model and we find that they follow similar paths in their luminosity-temperature diagrams as known black hole binaries. Also, the `soft excess' reported for many of these ULXs at $\sim 0.2$ keV seems to roughly follow a trend $L_{soft} \propto T^{-3.5}$ when using the powerlaw plus a `cool' MCD model. The observed trend could arise from disc emission beamed by an outflowing wind around a $\sim 10$ solar mass black hole.