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The Soft State of Cygnus X-1 Observed with NuSTAR: A Variable Corona and a Stable Inner DiskWe 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 < approx. a* < approx. 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 approx.10deg-15deg. 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 approx. 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.
Document ID
20170003120
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Walton, D. J.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA, United States)
Tomsick, J. A.
(California Univ. Berkeley, CA, United States)
Madsen, K. K.
(California Inst. of Tech. Pasadena, CA, United States)
Grinberg, V.
(Massachusetts Inst. of Tech. Cambridge, MA, United States)
Barret, D.
(Toulouse Univ. France)
Boggs, S. E.
(California Univ. Berkeley, CA, United States)
Christensen, F. E.
(Technical Univ. of Denmark Lyngby, Denmark)
Clavel, M.
(California Univ. Berkeley, CA, United States)
Craig, W. W.
(California Univ. Berkeley, CA, United States)
Fabian, A. C.
(Cambridge Univ. Cambridge, United Kingdom)
Fuerst, F.
(California Inst. of Tech. Pasadena, CA, United States)
Hailey, C. J.
(Columbia Univ. New York, NY, United States)
Harrison, F. A.
(California Inst. of Tech. Pasadena, CA, United States)
Miller, J. M.
(Michigan Univ. Ann Arbor, MI, United States)
Parker, M. L.
(Cambridge Univ. Cambridge, United Kingdom)
Rahoui, F.
(Harvard Univ. Cambridge, MA, United States)
Stern, D.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA, United States)
Tao, L.
(California Inst. of Tech. Pasadena, CA, United States)
Zhang, W.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Wilms, J.
(Erlangen-Nuernberg Univ. Bamberg, Germany)
Date Acquired
April 6, 2017
Publication Date
July 25, 2016
Publication Information
Publication: The Astrophysical Journal
Publisher: The American Astronomical Society
Volume: 826
Issue: 1
ISSN: 2041-8205
e-ISSN: 2041-8213
Subject Category
Astrophysics
Report/Patent Number
GSFC-E-DAA-TN41072
Funding Number(s)
CONTRACT_GRANT: NAS8-03060
Distribution Limits
Public
Copyright
Other
Keywords
black hole physics

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