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 multi-colour disk (MCD) and a combination of these two models. We find that out of the 11 ULXs in our sample, 7 ULXs show a correlation between the luminosity and the photon index Γ (hereafter L −Γ correlation). Furthermore, out of the 7 ULXs that have the L −Γ correlation, 4 ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an L − Γ anti-correlation. The spectra of 4 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 (hereafter L −T) diagrams. Finally we show that the ’soft excess ’ reported for many of these ULXs at ∼ 0.2 keV seem to follow a trend L ∝ T −4 when modeled with a power-law plus a ’cool ’ MCD model. This is contrary to the expected L ∝ T 4 relation that is expected from theory and what is seen for many accreting BHBs. Keywords: X-ray − Black holes − X-ray sources − X-ray binaries PACS: 95.85.Nv, 97.60.Lf, 98.70.Qy, 97.80.Jp 1.