Evolution of the spectral curvature in the ultraluminous X-ray source Holmberg II X-1

Kajava, J. J. E.; Poutanen, J.; Farrell, S. A.; Grisé, F.; Kaaret, P.
Referencia bibliográfica

Monthly Notices of the Royal Astronomical Society, Volume 422, Issue 2, pp. 990-996.

Fecha de publicación:
5
2012
Número de autores
5
Número de autores del IAC
0
Número de citas
24
Número de citas referidas
22
Descripción
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.