  We present multiepoch spectropolarimetry of the superluminous interacting
Type IIn supernova SN2017hcc, covering 16 to 391 days after explosion. In our
first epoch we measure continuum polarization as high as 6%, making SN 2017hcc
the most intrinsically polarized SN ever reported. During the first 29 days of
coverage, when the polarization is strongest, the continuum polarization has a
wavelength dependence that rises toward blue wavelengths, and becomes
wavelength independent by day 45. The polarization strength drops rapidly
during the first month, even as the SN flux is still climbing to peak
brightness. Nonetheless, record-high polarization is maintained until day 68,
at which point the source polarization declines to 1.9%, comparable to peak
levels in previous well-studied SNe IIn. Thereafter the SN continues in
polarization decline, while exhibiting only minor changes in position angle on
the sky. The blue slope of the polarized continuum during the first month,
accompanied by short-lived polarized flux for Balmer emission, suggests that an
aspherical distribution of dust grains in pre-shock circumstellar material
(CSM) is echoing the SN IIn spectrum and strongly influencing the polarization,
while the subsequent decline during the wavelength-independent phase appears
broadly consistent with electron scattering near the SN/CSM interface. The
persistence of the polarization position angle between these two phases
suggests that the pre-existing CSM responsible for the dust scattering at early
times is part of the same geometric structure as the electron-scattering region
that dominates the polarization at later times. SN2017hcc appears to be yet
another, but much more extreme, case of aspherical yet well-ordered CSM in Type
IIn SNe, possibly resulting from pre-SN mass loss shaped by a binary progenitor
system.
