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The Soft State of Cygnus X-1 Observed with NuSTAR: A Variable Corona and a Stable Inner Disk

Walton, D. J. and Tomsick, J. A. and Madsen, K. K. and Grinberg, V. and Barret, D. and Boggs, S. E. and Christensen, F. E. and Clavel, M. and Craig, W. W. and Fabian, A. C. and Fuerst, F. and Hailey, C. J. and Harrison, F. A. and Miller, J. M. and Parker, M. L. and Rahoui, F. and Stern, D. and Tao, L. and Wilms, J. and Zhang, W. (2016) The Soft State of Cygnus X-1 Observed with NuSTAR: A Variable Corona and a Stable Inner Disk. Astrophysical Journal, 826 (1). Art. No. 87. ISSN 0004-637X. doi:10.3847/0004-637X/826/1/87.

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We present a multi-epoch hard X-ray analysis of Cygnus X-1 in its soft state based on four observations with the Nuclear Spectroscopic Telescope Array (NuSTAR). Despite the basic similarity of the observed spectra, there is clear spectral variability between epochs. To investigate this variability, we construct a model incorporating both the standard disk-corona continuum and relativistic reflection from the accretion disk, based on prior work on Cygnus X-1, and apply this model to each epoch independently. We find excellent consistency for the black hole spin and the iron abundance of the accretion disk, which are expected to remain constant on observational timescales. In particular, we confirm that Cygnus X-1 hosts a rapidly rotating black hole, 0.93 ≾ a* ≾ 0.96, in broad agreement with the majority of prior studies of the relativistic disk reflection and constraints on the spin obtained through studies of the thermal accretion disk continuum. Our work also confirms the apparent misalignment between the inner disk and the orbital plane of the binary system reported previously, finding the magnitude of this warp to be ~10°–15°. This level of misalignment does not significantly change (and may even improve) the agreement between our reflection results and the thermal continuum results regarding the black hole spin. The spectral variability observed by NuSTAR is dominated by the primary continuum, implying variability in the temperature of the scattering electron plasma. Finally, we consistently observe absorption from ionized iron at ~6.7 keV, which varies in strength as a function of orbital phase in a manner consistent with the absorbing material being an ionized phase of the focused stellar wind from the supergiant companion star.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Walton, D. J.0000-0001-5819-3552
Tomsick, J. A.0000-0001-5506-9855
Madsen, K. K.0000-0003-1252-4891
Grinberg, V.0000-0003-2538-0188
Barret, D.0000-0002-0393-9190
Boggs, S. E.0000-0001-9567-4224
Christensen, F. E.0000-0001-5679-1946
Clavel, M.0000-0003-0724-2742
Fabian, A. C.0000-0002-9378-4072
Fuerst, F.0000-0003-0388-0560
Harrison, F. A.0000-0003-2992-8024
Parker, M. L.0000-0002-8466-7317
Rahoui, F.0000-0001-7655-4120
Stern, D.0000-0003-2686-9241
Wilms, J.0000-0003-2065-5410
Zhang, W.0000-0002-1426-9698
Additional Information:© 2016 The American Astronomical Society. Received 2016 March 18; revised 2016 April 29; accepted 2016 May 11; published 2016 July 25. The authors would like to thank the referee for prompt and useful feedback. DB acknowledges financial support from the French Space Agency (CNES). VG acknowledges financial support provided by NASA through the Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 to MIT for Support of the Chandra X-Ray Center (CXC) and Science Instruments; CXC is operated by SAO for and on behalf of NASA under contract NAS8-03060. This research 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 ASI Science Data Center (ASDC, Italy) and Caltech (USA). Swift BAT transient monitor results are provided by the Swift BAT team. This research has also made use of MAXI data provided by RIKEN, JAXA, and the MAXI team. Facility: NuSTAR.
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Smithsonian Astrophysical Observatory (SAO)SV3-73016
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:black hole physics – X-rays: binaries – X-rays: individual (Cygnus X-1)
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2016-01
Issue or Number:1
Record Number:CaltechAUTHORS:20160517-183422492
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:67162
Deposited By: Joy Painter
Deposited On:18 May 2016 16:27
Last Modified:11 Nov 2021 00:28

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