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The Broadband Spectral Variability of Holmberg IX X-1

Walton, D. J. and Fürst, F. and Harrison, F. A. and Middleton, M. J. and Fabian, A. C. and Bachetti, M. and Barret, D. and Miller, J. M. and Ptak, A. and Rana, V. and Stern, D. and Tao, L. (2017) The Broadband Spectral Variability of Holmberg IX X-1. Astrophysical Journal, 839 (2). Art. No. 105. ISSN 1538-4357.

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We present results from four new broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 (L_X > 10^(40) erg s^(−1)), performed by Suzaku and NuSTAR in coordination. Combined with the archival data, we now have broadband observations of this remarkable source from six separate epochs. Two of these new observations probe lower fluxes than seen previously, allowing us to extend our knowledge of the broadband spectral variability exhibited. The spectra are well fit by two thermal blackbody components that dominate the emission below 10 keV, as well as a steep (Γ ~ 3.5) power-law tail that dominates above ~15 keV. Remarkably, while the 0.3–10.0 keV flux varies by a factor of ~3 between all these epochs, the 15–40 keV flux varies by only ~20%. Although the spectral variability is strongest in the ~1–10 keV band, both of the thermal components are required to vary when all epochs are considered. We also revisit the search for iron absorption features by leveraging the high-energy NuSTAR data to improve our sensitivity to extreme velocity outflows in light of the ultra-fast outflow recently detected in NGC 1313 X-1. Iron absorption from a similar outflow along our line of sight can be ruled out in this case. We discuss these results in the context of super-Eddington accretion models that invoke a funnel-like geometry for the inner flow, and propose a scenario in which we have an almost face-on view of a funnel that expands to larger radii with increasing flux, resulting in an increasing degree of geometrical collimation for the emission from intermediate-temperature regions.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Walton, D. J.0000-0001-5819-3552
Fürst, F.0000-0003-0388-0560
Harrison, F. A.0000-0003-2992-8024
Middleton, M. J.0000-0002-8183-2970
Fabian, A. C.0000-0002-9378-4072
Bachetti, M.0000-0002-4576-9337
Barret, D.0000-0002-0393-9190
Ptak, A.0000-0001-5655-1440
Rana, V.0000-0003-1703-8796
Stern, D.0000-0003-2686-9241
Additional Information:© 2017 The American Astronomical Society. Received 2016 October 19; revised 2017 March 15; accepted 2017 March 17; published 2017 April 21. The authors would like to thank the anonymous reviewer, who provided useful suggestions for improving the final manuscript, and also Tim Roberts for useful discussion. D.J.W. and M.J.M. acknowledge support from STFC through Ernest Rutherford fellowships, A.C.F. acknowledges support from ERC Advanced Grant 340442, and D.B. acknowledges 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, XMM, Suzaku.
Group:NuSTAR, Space Radiation Laboratory, Astronomy Department
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)UNSPECIFIED
European Research Council (ERC)340442
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:black hole physics – X-rays: binaries – X-rays: individual (Holmberg IX X-1)
Issue or Number:2
Record Number:CaltechAUTHORS:20170421-141946967
Persistent URL:
Official Citation:D. J. Walton et al 2017 ApJ 839 105
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76824
Deposited By: Tony Diaz
Deposited On:21 Apr 2017 22:00
Last Modified:09 Mar 2020 13:19

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