SN 2016esw: a luminous Type II supernova observed within the first day after the explosion

TL;DR

SN 2016esw is a luminous Type II supernova observed within a day of explosion, showing unique spectral and light-curve features that suggest interaction with circumstellar matter and challenge its use for cosmology.

Contribution

This study provides detailed early observations and analysis of SN 2016esw, highlighting its high luminosity, atypically flat plateau, and evidence of ejecta-CSM interaction, expanding understanding of luminous Type II supernovae.

Findings

SN 2016esw has a peak magnitude of -18.36 mag.

The supernova exhibits a flatter plateau slope than typical SN II.

Spectral features indicate interaction with circumstellar matter.

Abstract

We present photometry, spectroscopy, and host-galaxy integral-field spectroscopy of the Type II supernova (SN) 2016esw in CGCG~229-009 from the first day after the explosion up to 120 days. Its light-curve shape is similar to that of a typical SN II; however, SN 2016esw is near the high-luminosity end of the SN II distribution, with a peak of $M^{\rm max}_{V}=-18.36$ mag. The $V$-band light curve exhibits a long recombination phase for a SN II (similar to the long-lived plateau of SN 2004et). Considering the well-known relation between the luminosity and the plateau decline rate, SN 2016esw should have a $V$-band slope of $\sim 2.10$ mag (100 days)$^{-1}$; however, SN 2016esw has a substantially flatter plateau with a slope of $1.01\pm 0.26$ mag (100 days)$^{-1}$, perhaps indicating that interacting Type II supernovae are not useful for cosmology. At 19.5 days post-explosion, the spectrum presents a boxy H$\alpha$ emission line with flat absorption profiles, suggesting interaction between the ejecta and circumstellar matter. Finally, based on the spectral properties, SN 2016esw shows similarities with the luminous and interacting SN 2007pk at early epochs, particularly in terms of observable line features and their evolution.