The aspherical explosions of the 03fg-like Type Ia supernovae 2021zny and 2022ilv revealed by polarimetry

TL;DR

This study uses polarimetry to reveal complex aspherical ejecta structures in two 03fg-like Type Ia supernovae, supporting the hypothesis that they originate from violent white dwarf mergers.

Contribution

First polarimetric observations of 03fg-like SNe 2021zny and 2022ilv demonstrating aspherical ejecta structures, supporting the violent merger origin hypothesis.

Findings

High intrinsic polarization (~1-2%) indicating aspherical ejecta.

Complex polarization angles suggesting multiple components.

Evidence supporting violent white dwarf merger scenario.

Abstract

A peculiar subtype of Type Ia supernovae (SNe), 03fg-like (super-Chandrasekhar) SNe, show different observational properties from prototypical Type Ia SNe, typically having high luminosity at the light-curve peak, low expansion velocities, and strong carbon features. The origin of this class of Type Ia SNe has been actively debated. Recent nebular-phase infrared observations of the 03fg-like Type Ia SN 2022pul using the James Webb Space Telescope revealed large-scale asymmetries in the ejecta and the presence of the strong [Ne II] line at 12.81 $\mu$m, suggesting a violent merger of two white dwarfs as its origin. Polarimetry is another powerful tool to study overall ejecta asymmetries of spatially-unresolved SNe. Here, we aim to check the universality of the violent merger scenario as the origin of the 03fg-like Type Ia SNe, by studying their explosion geometries using polarimetry. In this letter, we present imaging-polarimetric observations of the two 03fg-like Type Ia SNe 2021zny and 2022ilv. SNe 2021zny and 2022ilv show high intrinsic polarization ($\sim1$ % -$\sim2$ %), which might be composed of multiple components with different polarization angles. This indicates that they have complex aspherical structures in their ejecta, supporting the violent merger scenario for their origin. Our observations provide the first clear evidence from polarimetry for such aspherical structures.