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Abstract

We present a new analysis of NuSTAR and Suzaku observations of the black hole
Cygnus X-1 in the intermediate state. The analysis uses kerrC, a new model for
analyzing spectral and spectropolarimetric X-ray observations of black holes.
kerrC builds on a large library of simulated black holes in X-ray binaries. The
model accounts for the X-ray emission from a geometrically thin, optically
thick accretion disk, the propagation of the X-rays through the curved black
hole spacetime, the reflection off the accretion disk, and the Comptonization
of photons in coronae of different 3-D shapes and physical properties before
and after the reflection. We present the results from using kerrC for the
analysis of archival NuSTAR and Suzaku observations taken on May 27-28, 2015.
The best wedge-shaped corona gives a better fit than the cone-shaped corona.
Although we included cone-shaped coronae in the funnel regions above and below
the black hole to resemble to some degree the common assumption of a compact
lamppost corona hovering above and/or below the black hole, the fit chooses a
very large version of this corona that makes it possible to Comptonize a
sufficiently large fraction of the accretion disk photons to explain the
observed power law emission. The analysis indicates a dimensionless black hole
spin parameter a between 0.86 and 0.92. The kerrC model provides new insights
about the radial distribution of the energy flux of returning and coronal
emission irradiating the accretion disk. kerrC furthermore predicts small
polarization fractions around 1% in the 2-8 keV energy range of the recently
launched Imaging X-ray Polarimetry Explorer.