J. J. E. Kajava, J. Poutanen, S. A. Farrell, F. Grisé, P. Kaaret
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.
View original:
http://arxiv.org/abs/1202.1102
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