  We present a multi-epoch hard X-ray analysis of Cygnus X-1 in its soft state
based on four observations with 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 NuSTAR 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 ~10-15deg. 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 ~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.
