SN 2016gsd: An unusually luminous and linear type II supernova with high velocities

TL;DR

SN 2016gsd is an exceptionally luminous and linearly evolving Type II supernova with high velocities, likely influenced by ejecta interaction with asymmetric circumstellar material, challenging existing models of supernova luminosity and evolution.

Contribution

This study presents detailed observations of SN 2016gsd, highlighting its unusual luminosity, velocity profile, and spectral features, and proposes ejecta-CSM interaction as a key factor in its properties.

Findings

SN 2016gsd is one of the brightest Type II SNe observed.

The supernova exhibits high velocities and a linear decline in spectral line velocities.

Interaction with asymmetric circumstellar material likely influences its luminosity and spectral features.

Abstract

We present observations of the unusually luminous Type II supernova (SN) 2016gsd. With a peak absolute magnitude of V = $-$19.95 $\pm$ 0.08, this object is one of the brightest Type II SNe, and lies in the gap of magnitudes between the majority of Type II SNe and the superluminous SNe. Its light curve shows little evidence of the expected drop from the optically thick phase to the radioactively powered tail. The velocities derived from the absorption in H$\alpha$ are also unusually high with the blue edge tracing the fastest moving gas initially at 20000 km s$^{-1}$, and then declining approximately linearly to 15000 km s$^{-1}$ over $\sim$100 d. The dwarf host galaxy of the SN indicates a low-metallicity progenitor which may also contribute to the weakness of the metal lines in its spectra. We examine SN 2016gsd with reference to similarly luminous, linear Type II SNe such as SNe 1979C and 1998S, and discuss the interpretation of its observational characteristics. We compare the observations with a model produced by the JEKYLL code and find that a massive star with a depleted and inflated hydrogen envelope struggles to reproduce the high luminosity and extreme linearity of SN 2016gsd. Instead, we suggest that the influence of interaction between the SN ejecta and circumstellar material can explain the majority of the observed properties of the SN. The high velocities and strong H$\alpha$ absorption present throughout the evolution of the SN may imply a circumstellar medium configured in an asymmetric geometry.