SN 2020bqj: a Type Ibn supernova with a long lasting peak plateau

TL;DR

SN 2020bqj is a peculiar Type Ibn supernova with an unusually long-lasting peak plateau, likely powered by ejecta-CSM interaction, indicating a transitional progenitor between luminous blue variable and Wolf-Rayet stars.

Contribution

This paper presents the discovery and detailed analysis of SN 2020bqj, highlighting its unique long peak plateau and proposing a novel progenitor scenario involving a transitional massive star.

Findings

SN 2020bqj exhibits a long-lasting 40-day peak plateau unlike typical SNe Ibn.

Modeling suggests shock heating from ejecta-CSM interaction powers its lightcurve.

Similarity to SN 2011hw indicates a transitional progenitor phase between LBV and Wolf-Rayet stars.

Abstract

Context: Type Ibn supernovae are a rare class of stripped envelope supernovae interacting with a helium-rich CSM. The majority of the SNe Ibn reported display a surprising homogeneity in their fast lightcurves and starforming hosts. Aims: We present the discovery and study of SN 2020bqj (ZTF20aalrqbu), a SN Ibn with a long-duration peak plateau lasting 40 days and hosted by a faint low-mass galaxy. We aim to explain its peculiar properties using an extensive data set. Methods: We compare the evolution of SN 2020bqj with SNe Ibn from the literature. We fit the bolometric and multi-band lightcurves with different powering mechanism models. Results: The risetime, peak magnitude and spectral features of SN 2020bqj are consistent with those of most SNe Ibn, but the SN is a clear outlier based on its bright, long-lasting peak plateau and low host mass. We show through modeling that the lightcurve can be powered predominantly by shock heating from the interaction of the SN ejecta and a dense CSM. The peculiar Type Ibn SN 2011hw is a close analog to SN 2020bqj, suggesting a similar progenitor and CSM scenario. In this scenario a very massive progenitor star in the transitional phase between a luminous blue variable and a compact Wolf-Rayet star undergoes core-collapse, embedded in a dense helium-rich CSM with an elevated opacity compared to normal SNe Ibn, due to the presence of residual hydrogen. This scenario is consistent with the observed properties of SN 2020bqj and the modeling results. Conclusions: SN 2020bqj is a compelling example of a transitional SN Ibn/IIn based on not only its spectral features, but also its lightcurve, host galaxy properties and the inferred progenitor properties. The strong similarity with SN 2011hw suggests this subclass may be the result of a progenitor in a stellar evolution phase that is distinct from those of progenitors of regular SNe Ibn.