Optical polarization and spectral properties of the H-poor superluminous supernovae SN 2021bnw and SN 2021fpl

TL;DR

This study combines optical data and polarimetry to analyze the physical and spectral properties of two H-poor superluminous supernovae, revealing differences in their spectral evolution and polarization, and comparing their symmetry and magnetar parameters.

Contribution

It provides new polarimetric and spectral analysis of SN 2021bnw and SN 2021fpl, including polarization detections and spectral transition timing, expanding understanding of H-poor SLSN properties.

Findings

SN 2021bnw shows no polarization departure, indicating symmetry.

SN 2021fpl exhibits >3σ polarization, suggesting asymmetry.

Spectral transition in SN 2021fpl occurs earlier than in SN 2015bn.

Abstract

New optical photometric, spectrocopic and imaging polarimetry data are combined with publicly available data to study some of the physical properties of the two H-poor superluminous supernovae (SLSN) SN 2021bnw and SN 2021fpl. For each SLSN, the best-fit parameters obtained from the magnetar model with \texttt{MOSFiT} do not depart from the range of parameter obtained on other SLSNe discussed in the literature. A spectral analysis with \texttt{SYN++} shows that SN 2021bnw is a W Type, Fast evolver, while SN 2021fpl is a 15bn Type, Slow evolver. The analysis of the polarimetry data obtained on SN 2021fpl at four epochs (+1.8, +20.6, +34.1 and +43.0 days, rest-frame) shows $> 3\sigma$ polarization detections in the range 0.8--1 $\%$. A comparison of the spectroscopy data suggests that SN 2021fpl underwent a spectral transition a bit earlier than SN 2015bn, during which, similarly, it could have underwent a polarization transition. The analysis of the polarimetry data obtained on SN 2021bnw do not show any departure from symmetry of the photosphere at an empirical diffusion timescale of $\approx$ 2 (+81.1 days rest-frame). This result is consistent with those on the sample of W Type SLSN observed at empirical diffusion timescale $\le$ 1 with that technique, even though it is not clear the effect of limited spectral windows varying from one object to the other. Measurements at higher empirical diffusion timescale may be needed to see any departure from symmetry as it is discussed in the literature for SN 2017egm.