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ABSTRACT Optical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np from −14. 5 to +14. 5 d relative to B-band maximum detected an intrinsic continuum polarization (pcont) of 0. 21 ± 0. 09 per cent at the first epoch. Between days −11. 5 and +0. 5, pcont remained ∼0 and by day +14. 5 was again significant at 0. 19 ± 0. 10 per cent. Not considering the first epoch, the dominant axis of Si\, { II} λ6355 was roughly constant staying close the continuum until both rotated in opposite directions on day +14. 5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch pcont ≈ 0. 2 per cent nevertheless suggests a separate structure with an axis ratio ∼2 in the outer carbon-rich (3. 5–4) × 10−3 M⊙. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the Ca\, { II} near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-centre delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The pcont and the absorptions due to Si\, { II} λ6355 and Ca\, { II} NIR3 form in the same region of the extended photosphere, with an interplay between line occultation and thermalization producing p. Small-scale polarization features may be due to small-scale structures, but many could be related to atomic patterns of the quasi-continuum; they hardly have an equivalent in the total-flux spectra. We compare SN 2019np to other SNe and develop future objectives and strategies for SN Ia spectropolarimetry.
Hoêflich et al. (Mon,) studied this question.