Evolution of the spectral curvature in the ULX Holmberg II X-1

Details Evolution of the spectral curvature in the ULX Holmberg II X-1 Evolution of the spectral curvature in the ULX Holmberg II X-1

2012-02-06

arxiv.org/abs/1202.1102v1

Astronomy and Astrophysics

Astrophysics

High Energy Astrophysical Phenomena

7 pages, 3 figures and 2 tables. Accepted for publication in MNRAS

Scientific paper

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.

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