Understanding the extreme luminosity of DES14X2fna

TL;DR

DES14X2fna is a highly luminous, fast-declining type IIb supernova with unusual light curve features that challenge standard models and suggest additional energy sources like circumstellar interaction or a magnetar.

Contribution

This paper presents detailed observations and modeling of DES14X2fna, revealing that standard radioactive decay models cannot explain its light curve, and explores implications for supernova classification in cosmology.

Findings

Standard $^{56}$Ni decay models fail to fit the supernova's light curve.

Interaction with circumstellar material or a magnetar improves model fits.

A supernova similar to DES14X2fna can be misclassified as a SN Ia in photometric surveys.

Abstract

We present DES14X2fna, a high-luminosity, fast-declining type IIb supernova (SN IIb) at redshift $z=0.0453$, detected by the Dark Energy Survey (DES). DES14X2fna is an unusual member of its class, with a light curve showing a broad, luminous peak reaching $M_r\simeq-19.3$ mag 20 days after explosion. This object does not show a linear decline tail in the light curve until $\simeq$60 days after explosion, after which it declines very rapidly (4.38$\pm$0.10 mag 100 d$^{-1}$ in $r$-band). By fitting semi-analytic models to the photometry of DES14X2fna, we find that its light curve cannot be explained by a standard $^{56}$Ni decay model as this is unable to fit the peak and fast tail decline observed. Inclusion of either interaction with surrounding circumstellar material or a rapidly-rotating neutron star (magnetar) significantly increases the quality of the model fit. We also investigate the possibility for an object similar to DES14X2fna to act as a contaminant in photometric samples of SNe Ia for cosmology, finding that a similar simulated object is misclassified by a recurrent neural network (RNN)-based photometric classifier as a SN Ia in $\sim$1.1-2.4 per cent of cases in DES, depending on the probability threshold used for a positive classification.