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We present multiepoch spectropolarimetry of the superluminous interactingType IIn supernova SN2017hcc, covering 16 to 391 days after explosion. In ourfirst epoch we measure continuum polarization as high as 6%, making SN 2017hccthe most intrinsically polarized SN ever reported. During the first 29 days ofcoverage, when the polarization is strongest, the continuum polarization has awavelength dependence that rises toward blue wavelengths, and becomeswavelength independent by day 45. The polarization strength drops rapidlyduring the first month, even as the SN flux is still climbing to peakbrightness. Nonetheless, record-high polarization is maintained until day 68,at which point the source polarization declines to 1.9%, comparable to peaklevels in previous well-studied SNe IIn. Thereafter the SN continues inpolarization decline, while exhibiting only minor changes in position angle onthe sky. The blue slope of the polarized continuum during the first month,accompanied by short-lived polarized flux for Balmer emission, suggests that anaspherical 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 appearsbroadly consistent with electron scattering near the SN/CSM interface. Thepersistence of the polarization position angle between these two phasessuggests that the pre-existing CSM responsible for the dust scattering at earlytimes is part of the same geometric structure as the electron-scattering regionthat dominates the polarization at later times. SN2017hcc appears to be yetanother, but much more extreme, case of aspherical yet well-ordered CSM in TypeIIn SNe, possibly resulting from pre-SN mass loss shaped by a binary progenitorsystem.

Record-breaking polarization from the interacting superluminous supernova 2017hcc