An Optical and Near-Infrared Study of the Type Ia/IIn Supernova PS15si

TL;DR

This study presents optical and near-infrared observations of supernova PS15si, reclassifying it as a Type Ia with circumstellar interaction, and analyzes its spectral features, luminosity, and possible CSM interactions.

Contribution

It provides the first detailed spectral and photometric analysis of PS15si, suggesting a reclassification and insights into CSM interaction effects in Type Ia supernovae.

Findings

PS15si is best classified as a Type Ia SN with CSM interaction.

Spectral analysis aligns PS15si with SN 1991T-like Type Ia SNe.

Evidence of possible rebrightening due to CSM interaction at late times.

Abstract

We present optical/near-infrared spectroscopy and photometry of the supernova (SN) PS15si. This object was originally identified as a Type IIn SN, but here we argue that it should be reclassified as a Type Ia SN with narrow hydrogen lines originating from interaction with circumstellar matter (CSM; i.e., SN Ia/IIn or SN Ia-CSM). Based on deep nondetections 27 days before discovery, we infer that this SN was discovered around or slightly before optical maximum, and we estimate the approximate time that it reached R-band maximum based on comparison with other SNe Ia/IIn. In terms of spectral morphology, we find that PS15si can be matched to a range of SN Ia spectral types, although SN 1991T-like SNe Ia provides the most self-consistent match. While this observation agrees with analysis of most other SNe Ia/IIn, we find that the implied CSM luminosity is too low to account for the overall luminosity of the SN at a time when the CSM should outshine the underlying SN by a few magnitudes. We infer that the similarity between PS15si and the hot, overluminous, high-ionisation spectrum of SN 1991T is a consequence of a spectrum that originates in ejecta layers that are heated by ultraviolet/X-ray radiation from CSM interaction. In addition, PS15si may have rebrightened over a short timescale in the B and V bands around 85 days after discovery, perhaps indicating that the SN ejecta are interacting with a local enhancement in CSM produced by clumps or a shell at large radii.