Spectropolarimetry of the Thermonuclear Supernova 2021rhu: High Calcium Polarization 79 Days After Peak Luminosity

We report spectropolarimetric observations of the Type Ia supernova (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 $3890 \pm 93$ Angstroms and reaching a level of $p_{\rm max}=1.78% \pm 0.02$% 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 sightline to the SN. After removing the interstellar polarization, we found a pronounced increase of the polarization in the CaII near-infrared triplet, from $\sim$0.3% at day $-$7 to $\sim$2.5% at day +79. No temporal evolution in high-resolution flux spectra across the NaID and CaIIH&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 CaII 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.