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Published June 9, 2015 | Published
Journal Article Open

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 2013 September. 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 Xray 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 L_X (8.1 0.1) 1039 =±× erg s^(−1), typical for Holmberg II X^(-1), with the majority of this flux (∼90%) emitted below 10 keV.

Additional Information

© 2015 American Astronomical Society. Received 2015 March 30; accepted 2015 April 24; published 2015 June 9. The authors would like to thank the referee for the positive feedback, which helped improve the clarity of the final manuscript. M.B. and D.B. acknowledge financial support from the French Space Agency (CNES). This research has made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL and has utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy) and Caltech (USA). This research has also made use of data obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States, and with Suzaku, a collaborative mission between the space agencies of Japan (JAXA) and the USA (NASA). Facilites: NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, XMM - Newton X-Ray Multimirror Mission satellite, Suzaku - Suzaku (ASTRO-EII).

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August 22, 2023
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