期刊
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
卷 422, 期 2, 页码 990-996出版社
WILEY-BLACKWELL
DOI: 10.1111/j.1365-2966.2012.20671.x
关键词
accretion, accretion discs; black hole physics; X-rays: binaries; X-rays: galaxies; X-rays: individual: Holmberg II X-1
资金
- Emil Aaltonen Foundation
- Finnish Graduate School in Astronomy and Space Physics
- Academy of Finland [127512]
- ARC
- UK STFC
- NASA [NNX10AF86G]
- ESA Member States
- [DP110102889]
- NASA [NNX10AF86G, 134394] Funding Source: Federal RePORTER
- Academy of Finland (AKA) [127512, 127512] Funding Source: Academy of Finland (AKA)
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.
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