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 disc (MCD) and a combination
of these two models. We find that 7 ULXs show a correlation between the
luminosity Lx and the photon index Gamma. Furthermore, 4 out of these 7 ULXs
also show spectral pivoting in the observed energy band. We also find that two
ULXs show an Lx-Gamma 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 diagrams. Finally we show that the `soft excess'
reported for many of these ULXs at about 0.2 keV seems to roughly follow a
trend Lsoft \propto T^{-3.5} when modelled with a power-law plus a `cool' MCD
model. This is contrary to the L \propto 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 about 10
solar mass black hole.
