Spectral variability of ultraluminous X-ray sources

Understand

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 Gamma (hereafter L-Gamma correlation).
• Furthermore, out of the 7 ULXs that have the L-Gamma correlation, 4 ULXs also show spectral pivoting in the observed energy band.