  We use XMM-Newton and Swift data to study spectral variability in the
ultraluminous X-ray source (ULX), Holmberg IX X-1. The source luminosity varies
by a factor 3-4, giving rise to corresponding spectral changes which are
significant, but subtle, and not well tracked by a simple hardness ratio.
Instead, we co-add the Swift data in intensity bins and do full spectral
fitting with disc plus thermal Comptonisation models. All the data are
well-fitted by a low temperature, optically thick Comptonising corona, and the
variability can be roughly characterised by decreasing temperature and
increasing optical depth as the source becomes brighter, as expected if the
corona is becoming progressively mass loaded by material blown off the
super-Eddington inner disc. This variability behaviour is seen in other ULX
which have similar spectra, but is opposite to the trend seen in ULX with much
softer spectra. This supports the idea that there are two distinct physical
regimes in ULXs, where the spectra go from being dominated by a disc-corona to
being dominated by a wind.

High Energy Astrophysical Phenomena