Evolution of the spectral curvature in the ULX 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 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.

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

References in corpus (7)

• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• On the weakness of disc models in bright ULXs
• Identification of the X-Ray Thermal Dominant State in an Ultraluminous X-Ray Source in M82
• Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources
• The Ultraluminous X-ray Sources near the Center of M82
• X-ray spectral state is not correlated with luminosity in Holmberg II X-1

Cited by in corpus (10)

• ULX spectra revisited: Accreting, highly magnetized neutron stars as the engines of ultraluminous X-ray sources
• NuSTAR, XMM-Newton and Suzaku Observations of the Ultraluminous X-ray Source Holmberg II X-1
• Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources
• A deficit of ultraluminous X-ray sources in luminous infrared galaxies
• Patchy Accretion Disks in Ultraluminous X-ray Sources
• Revealing the nature of the ULX and X-ray population of the spiral galaxy NGC 4088
• An extreme Ultraluminous X-ray source X-1 in NGC 5055
• Modelling multiwavelength emission of Ultra-luminous X-ray Sources accreting above the Eddington limit
• Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor
• Simultaneous X-ray/UV observations of ultraluminous X-ray source Holmberg II X-1 with Indian space mission AstroSat