Evolution of the spectral curvature in the ultraluminous X-ray source Holmberg II X-1
Abstract
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2012
- DOI:
- 10.1111/j.1365-2966.2012.20671.x
- arXiv:
- arXiv:1202.1102
- Bibcode:
- 2012MNRAS.422..990K
- Keywords:
-
- accretion;
- accretion discs;
- black hole physics;
- X-rays: binaries;
- X-rays: galaxies;
- X-rays: individual: Holmberg II X-1;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Cosmology and Extragalactic Astrophysics
- E-Print:
- 7 pages, 3 figures and 2 tables. Accepted for publication in MNRAS