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We present a multi-epoch hard X-ray analysis of Cygnus X-1 in its soft statebased on four observations with NuSTAR. Despite the basic similarity of theobserved spectra, there is clear spectral variability between epochs. Toinvestigate this variability, we construct a model incorporating both thestandard disk-corona continuum and relativistic reflection from the accretiondisk, based on prior NuSTAR work on Cygnus X-1, and apply this model to eachepoch independently. We find excellent consistency for the black hole spin, andthe iron abundance of the accretion disk, which are expected to remain constanton observational timescales. In particular, we confirm that Cygnus X-1 hosts arapidly rotating black hole, 0.93<a*<0.96, in broad agreement with the majorityof prior studies of the relativistic disk reflection and constraints on thespin obtained through studies of the thermal accretion disk continuum. Our workalso confirms the apparent misalignment between the inner disk and the orbitalplane of the binary system reported previously, finding the magnitude of thiswarp to be ~10-15deg. This level of misalignment does not significantly change(and may even improve) the agreement between our reflection results and thethermal continuum results regarding the black hole spin. The spectralvariability observed by NuSTAR is dominated by the primary continuum, implyingvariability in the temperature of the scattering electron plasma. Finally, weconsistently observe absorption from ionized iron at ~6.7 keV, which varies instrength as a function of orbital phase in a manner consistent with theabsorbing material being an ionized phase of the focused stellar wind from thesupergiant companion star.

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