Evolution of the spectral curvature in the ULX Holmberg II X-1
  Ultraluminous X-ray sources (ULXs) are interesting systems as they can host
intermediate mass black holes. Alternatively, ULXs can represent stellar-mass
black holes accreting at super-Eddington rates. Recently spectral curvature or
breaks at energies above a few keV have been detected in high quality ULX
spectra. These spectral features have been taken as evidence against the
intermediate-mass black hole case. In this paper, we report on a new XMM-Newton
observation of the ULX Holmberg II X-1 that also shows a clear spectral break
at approximately 4 keV. This observation was performed during a low luminosity
state of the system and by comparing this new data to a high luminosity state
XMM-Newton observation, we can conclude that the spectral break energy
increases with luminosity. This behaviour is different to a ULX in the Holmberg
IX galaxy,where an opposite trend between the luminosity and the spectral break
energy has been claimed. We discuss mechanisms that could explain this complex
behaviour.
