Bibcode
Kajava, J. J. E.; Poutanen, J.; Farrell, S. A.; Grisé, F.; Kaaret, P.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 422, Issue 2, pp. 990-996.
Advertised on:
5
2012
Citations
24
Refereed citations
22
Description
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 these 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 from 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.