The soft state of Cygnus X-1 observed with nustar: A variable corona and a stable inner disk

Journal article
(Original article)

Publication Details

Author(s): Walton DJ, Tomsick JA, Madsen KK, Grinberg V, Barret D, Boggs SE, Christensen FE, Clavel M, Craig WW, Fabian AC, Fuerst F, Hailey CJ, Harrison FA, Miller JM, Parker ML, Rahoui F, Stern D, Tao L, Wilms J, Zhang W
Journal: Astrophysical Journal
Publisher: Institute of Physics Publishing
Publication year: 2016
Volume: 826
Journal issue: 1
ISSN: 0004-637X
eISSN: 1538-4357


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.

FAU Authors / FAU Editors

Wilms, Jörn Prof. Dr.
Professur für Astronomie und Astrophysik

External institutions with authors

California Institute of Technology (Caltech)
Columbia University
European Southern Observatory (ESO)
Massachusetts Institute of Technology (MIT)
National Aeronautics and Space Administration (NASA)
Technical University of Denmark / Danmarks Tekniske Universitet (DTU)
University of California, Berkeley
University of Cambridge
University of Michigan
University of Toulouse

How to cite

Walton, D.J., Tomsick, J.A., Madsen, K.K., Grinberg, V., Barret, D., Boggs, S.E.,... 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).

Walton, D. J., et al. "The soft state of Cygnus X-1 observed with nustar: A variable corona and a stable inner disk." Astrophysical Journal 826.1 (2016).


Last updated on 2019-23-08 at 08:32