NuSTAR, XMM-Newton and Suzaku Observations of the Ultraluminous X-ray Source Holmberg II X-1

Abstract

We present the first broadband 0.3-25.0 kev X-ray observations of the bright ultraluminous X-ray source (ULX) Holmberg II X-1, performed by NuSTAR, XMM-Newton and Suzaku in September 2013. The NuSTAR data provide the first observations of Holmberg II X-1 above 10 keV, and reveal a very steep high-energy spectrum, similar to other ULXs observed by NuSTAR to date. These observations further demonstrate that ULXs exhibit spectral states that are not typically seen in Galactic black hole binaries. Comparison with other sources implies that Holmberg II X-1 accretes at a high fraction of its Eddington accretion rate, and possibly exceeds it. The soft X-ray spectrum (E<10 keV) appears to be dominated by two blackbody-like emission components, the hotter of which may be associated with an accretion disk. However, all simple disk models under-predict the NuSTAR data above ~10 keV and require an additional emission component at the highest energies probed, implying the NuSTAR data does not fall away with a Wien spectrum. We investigate physical origins for such an additional high-energy emission component, and favor a scenario in which the excess arises from Compton scattering in a hot corona of electrons with some properties similar to the very-high state seen in Galactic binaries. The observed broadband 0.3-25.0 keV luminosity inferred from these epochs is Lx = (8.1+/-0.1)e39 erg/s, typical for Holmberg II X-1, with the majority of the flux (~90%) emitted below 10 keV.

8 pages, 2 figures, accepted for publication in ApJ

References in corpus (17)

• X-ray Properties of Black-Hole Binaries
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• The Nature of SS433 and the Ultraluminous X-ray Sources
• A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
• A 400 solar mass black hole in the Ultraluminous X-ray source M82 X-1 accreting close to its Eddington limit
• A spectral-timing model for ULXs in the super-critical regime
• X-ray observations of ultraluminous X-ray sources
• The Broad-band X-ray Spectrum of IC 4329A from a Joint NuSTAR/Suzaku Observation
• NuSTAR and XMM-Newton observations of the extreme ultraluminous X-ray source NGC 5907 ULX1: A Vanishing Act
• X-ray spectral state is not correlated with luminosity in Holmberg II X-1
• Determining the Nature of the SS 433 Binary Using an X-ray Spectrum During Eclipse
• Elemental Abundances of Nearby Galaxies through High Signal-to-Noise XMM-Newton Observations of ULXs
• Coupled radio and X-ray emission and evidence for discrete ejecta in the jets of SS 433
• Evolution of the spectral curvature in the ULX Holmberg II X-1
• Patchy Accretion Disks in Ultraluminous X-ray Sources

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