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.
@article{arxiv.1202.1102,
title = {Evolution of the spectral curvature in the ULX Holmberg II X-1},
author = {J. J. E. Kajava and J. Poutanen and S. A. Farrell and F. Grisé and P. Kaaret},
journal= {arXiv preprint arXiv:1202.1102},
year = {2012}
}
Comments
7 pages, 3 figures and 2 tables. Accepted for publication in MNRAS