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

Walton, D. J. and Middleton, M. J. and Rana, V. and Miller, J. M. and Harrison, F. A. and Fabian, A. C. and Bachetti, M. and Barret, D. and Boggs, S. E. and Christensen, F. E. and Craig, W. W. and Fürst, F. and Grefenstette, B. W. and Hailey, C. J. and Madsen, K. K. and Stern, D. and Zhang, W. (2015) NuSTAR, XMM-Newton, and Suzaku Observations of the Ultraluminous X-Ray Source Holmberg II X-1. Astrophysical Journal, 806 (1). Art No. 65. ISSN 0004-637X. doi:10.1088/0004-637X/806/1/65.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Walton, D. J.0000-0001-5819-3552
Middleton, M. J.0000-0002-8183-2970
Rana, V.0000-0003-1703-8796
Harrison, F. A.0000-0003-2992-8024
Fabian, A. C.0000-0002-9378-4072
Bachetti, M.0000-0002-4576-9337
Barret, D.0000-0002-0393-9190
Boggs, S. E.0000-0001-9567-4224
Christensen, F. E.0000-0001-5679-1946
Fürst, F.0000-0003-0388-0560
Grefenstette, B. W.0000-0002-1984-2932
Madsen, K. K.0000-0003-1252-4891
Stern, D.0000-0003-2686-9241
Zhang, W.0000-0002-1426-9698
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).
Group:NuSTAR, Space Radiation Laboratory, Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Japan Aerospace Exploration Agency (JAXA)UNSPECIFIED
Subject Keywords:black hole physics; X-rays: binaries; X-rays: individual (Holmberg II X-1)
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2015-58
Issue or Number:1
Record Number:CaltechAUTHORS:20160128-201524771
Persistent URL:
Official Citation:D. J. Walton et al 2015 ApJ 806 65
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64073
Deposited By: Joy Painter
Deposited On:29 Jan 2016 17:05
Last Modified:10 Nov 2021 23:25

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