The soft state of Cygnus X-1 observed with NuSTAR: a variable corona and a stable inner disk
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
- Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)
- Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 (United States)
- MIT Kavli Institute for Astrophysics and Space Research, MIT, 70 Vassar Street, Cambridge, MA 02139 (United States)
- Universite de Toulouse, UPS-OMP, IRAP, Toulouse (France)
- DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby (Denmark)
- Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA (United Kingdom)
- Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027 (United States)
- Department of Astronomy, University of Michigan, 1085 S. University Avenue, Ann Arbor, MI 48109-1107 (United States)
- European Southern Observatory, K. Schwarzschild-Strasse 2, D-85748 Garching bei Munchen (Germany)
- ECAP-Erlangen Centre for Astroparticle Physics, Sternwartstrasse 7, D-96049 Bamberg (Germany)
- NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
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{sup ∗}≲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.
- OSTI ID:
- 22868871
- Journal Information:
- Astrophysical Journal, Vol. 826, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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