The core normal Type Ia supernova 2019np-an overall spherical explosion with an aspherical surface layer and an aspherical 56Ni core

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 (p(cont)) of 0.21 +/- 0.09 per cent at the first epoch. Between days -11.5 and +0.5, p(cont) remained similar to 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 lambda 6355 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 p(cont) approximate to 0.2 per cent nevertheless suggests a separate structure with an axis ratio similar to 2 in the outer carbon-rich (3.5-4) x 10(-3) M-circle dot. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the 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 Ni-56 core. The p(cont) and the absorptions due to lambda 6355 and 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 SN2019np to other SNe and develop future objectives and strategies for SNIa spectropolarimetry.

Automated summary

Optical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np reveals a significant intrinsic continuum polarization, which remains stable throughout the observational period. The presence of a separate structure with a deviation from the continuum polarization axis is also suggested. The temporal evolution of the polarization spectra indicates an off-center delayed detonation scenario for SN2019np.