Spectropolarimetry of the thermonuclear supernova SN 2021rhu: High calcium polarization 79 days after peak luminosity

We report spectropolarimetric observations of the Type Ia supernova (SN) SN 2021rhu at four epochs: −7, +0, +36, and +79 days relative to its B-band maximum luminosity. A wavelength-dependent continuum polarization peaking at 3890 ±93Å and reaching a level of pmax = 1.78% 0.02 %was found. The peak of the polarization curve is bluer than is typical in the Milky Way, indicating a larger proportion of small dust grains along the sight line to the SN. After removing the interstellar polarization, we found a pronounced increase of the polarization in the CaII near-infrared triplet, from ∼0.3% at day −7to∼2.5% at day +79. No temporal evolution in highresolution flux spectra across the Na I D and Ca II H and K features was seen from days +39 to +74, indicating that the late-time increase in polarization is intrinsic to the SN as opposed to being caused by scattering of SN photons in circumstellar or interstellar matter. We suggest that an explanation for the late-time rise of the Ca II near-infrared triplet polarization may be the alignment of calcium atoms in a weak magnetic field through optical excitation/ pumping by anisotropic radiation from the SN.
The research of Y.Y. is supported through a Bengier-Winslow-Robertson Fellowship. M.B. acknowledges support from the Swedish Research Council (Reg. No. 2020-03330). A.V.F.'s group at U.C. Berkeley acknowledges generous support from the Miller Institute for Basic Research in Science (where A.V.F. was a Miller Senior Fellow), Sunil Nagaraj, Landon Noll, Gary and Cynthia Bengier, Clark and Sharon Winslow, Sanford Robertson, and many additional donors. L.G. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033, and the European Social Fund (ESF) "Investing in your future" under the 2019 Ramón y Cajal program RYC2019-027683-I and the PID2020-115253GA-I00 HOSTFLOWS project, from Centro Superior de Investigaciones Científicas (CSIC) under the PIE project 20215AT016, and the program Unidad de Excelencia María de Maeztu CEX2020-001058-M. P.H. acknowledges the support from the NSF project "Signatures of Type Ia Supernovae, New Physics, and Cosmology," grant AST-1715133. The supernova research by L.W. is supported by NSF award AST-1817099. M.R. is supported by the National Science Foundation Graduate Research Fellowship Program under grant DGE-1752134. J.C.W. and J.V. are supported by NSF grant AST-1813825. The research of J.M. is supported through a Royal Society University Research Fellowship.