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Extremely Strong Polarization Detected in Type IIn Supernova SN 2017hcc (ATLAS17lsn)

ATel #10911; Jon C. Mauerhan, Alexei V. Filippenko, Thomas G. Brink, Weikang Zheng (UC Berkeley)
on 31 Oct 2017; 03:36 UT
Credential Certification: Weikang Zheng (zwk@umich.edu)

Subjects: Optical, Supernovae

Referred to by ATel #: 10936, 11015

Spectropolarimetry of the interacting Type IIn SN 2017hcc (ATLAS17lsn) was obtained on 2017 Oct. 30 UT with the Kast spectrograph on the Shane 3-m telescope at Lick Observatory. We measure an integrated V-band continuum polarization of 4.84% +/- 0.02% at a position angle of 96.5 deg +/- 0.1 deg. This is, to our knowledge, the strongest continuum polarization ever reported for a supernova. Preliminary analysis indicates that the bulk of the polarization is intrinsic to the source and not the result of interstellar polarization (ISP); the object is at high Galactic latitude (-71 deg) with very little extinction from the Milky Way, and the Na-D absorption at the redshift of the host galaxy (z = 0.017) is not sufficiently strong for host ISP to explain the strong polarization. Moreover, the polarization exhibits only weak wavelength dependence, inconsistent with an interstellar Serkowski form. The polarization spectrum also exhibits strong depolarization in the cores of the narrow H-alpha and H-beta emission features, similar to what was observed for SN 2009ip (Mauerhan et al. 2014, MNRAS, 442, 1166). This characteristic is consistent with continuum polarization originating from electron scattering in a globally aspherical interaction region, perhaps resulting from a toroidal or disk-like distribution of circumstellar material (CSM), while the narrow depolarized cores of the Balmer lines probably result from photoionized pre-shock CSM.