SN 2015bp: adding to the growing population of transitional type Ia supernovae

TL;DR

This paper presents detailed photometric and spectroscopic observations of the transitional Type Ia supernova SN 2015bp, highlighting its unique light curve features, spectral evolution, and implications for progenitor models.

Contribution

It provides new observational data on SN 2015bp, classifies it as a transitional Type Ia supernova, and discusses its progenitor mass and explosion characteristics.

Findings

SN 2015bp shows rapidly declining light curves with Δm15(B)=1.72.

The I-band light curve exhibits a secondary maximum before B-band maximum.

Spectral features and velocity gradients suggest a sub-Chandrasekhar mass progenitor.

Abstract

Photometric and spectroscopic observations of type Ia supernova 2015bp are presented, spanning $\sim -6$ to $\sim +141$ days since $B$-band maximum. Also presented are unpublished HCT spectra of type Ia iPTF13ebh between $-11$ to +34 days since $B$-band maximum. SN 2015bp shows rapidly declining light curves with $\Delta m_{15}(B)=1.72 \pm 0.04$. The $I$-band light curve shows a clear secondary maximum and peaks before the $B$-band maximum, placing SN 2015bp in the transitional category of SNe Ia. The spectral evolution of SN 2015bp resembles other transitional SNe Ia rather than 1991bg-like events. The C~{\sc ii} $\lambda 6580$ feature is detected in both SN 2015bp and iPTF13ebh, though it is present till the epoch of $B$-band maximum in the case of SN 2015bp. The velocity gradients of Si~{\sc ii} $\lambda 6355$ place SN 2015bp and iPTF13ebh in the FAINT subclass, whereas pseudo-equivalent widths of Si~{\sc ii} features place them in the Cool (CL) subclass of SNe Ia. The bolometric light curve of SN 2015bp indicates that $\sim 0.2$ M$_{\odot}$ of $^{56}$Ni was synthesized in the explosion, with a total ejected mass of $\sim 0.9$ M$_{\odot}$, suggesting a sub-Chandrasekhar mass white dwarf progenitor.