  We present the first X-ray spectropolarimetric results for Cygnus X-1 in its
soft state from a campaign of five IXPE observations conducted during 2023
May-June. Companion multiwavelength data during the campaign are likewise
shown. The 2-8 keV X-rays exhibit a net polarization degree PD=1.99%+/-0.13%
(68% confidence). The polarization signal is found to increase with energy
across IXPE's 2-8 keV bandpass. The polarized X-rays exhibit an
energy-independent polarization angle of PA=-25.7+/-1.8 deg. East of North (68%
confidence). This is consistent with being aligned to Cyg X-1's AU-scale
compact radio jet and its pc-scale radio lobes. In comparison to earlier
hard-state observations, the soft state exhibits a factor of 2 lower
polarization degree, but a similar trend with energy and a similar (also
energy-independent) position angle. When scaling by the natural unit of the
disk temperature, we find the appearance of a consistent trendline in the
polarization degree between soft and hard states. Our favored polarimetric
model indicates Cyg X-1's spin is likely high (a* above ~0.96). The substantial
X-ray polarization in Cyg X-1's soft state is most readily explained as
resulting from a large portion of X-rays emitted from the disk returning and
reflecting off the disk surface, generating a high polarization degree and a
polarization direction parallel to the black hole spin axis and radio jet. In
IXPE's bandpass, the polarization signal is dominated by the returning
reflection emission. This constitutes polarimetric evidence for strong
gravitational lensing of X-rays close to the black hole.
