# Observations of A Fast-Expanding and UV-Bright Type Ia Supernova SN 2013gs

## TL;DR

SN 2013gs is a fast-expanding, UV-bright Type Ia supernova with unique spectral features and strong UV emission, providing insights into the diversity and origins of SNe Ia.

## Contribution

This study provides detailed optical and UV observations of SN 2013gs, highlighting its unusual UV brightness and spectral properties compared to normal SNe Ia.

## Key findings

- SN 2013gs shows unusually strong UV emission near maximum light.
- It has a higher expansion velocity (~13,000 km/s) than typical SNe Ia.
- SN 2013gs exhibits bluer uvw1-v color and similar Si and Fe line strengths compared to other fast-expanding SNe Ia.

## Abstract

In this paper, we present extensive optical and ultraviolet (UV) observations of the type Ia supernova (SN Ia) 2013gs discovered during the Tsinghua-NAOC Transient Survey. The photometric observations in the optical show that the light curves of SN 2013gs is similar to that of normal SNe Ia, with an absolute peak magnitude of $M_{B}$ = $-$19.25 $\pm$ 0.15 mag and a post-maximum decline rate $\Delta$m$_{15}$(B) = 1.00 $ \pm $ 0.05 mag. \emph{Gehrels Swift} UVOT observations indicate that SN 2013gs shows unusually strong UV emission (especially in the $uvw1$ band) at around the maximum light (M$_{uvw1}$ $\sim$ $-$18.9 mag). The SN is characterized by relatively weak Fe~{\sc ii} {\sc iii} absorptions at $\sim$ 5000{\AA} in the early spectra and a larger expansion velocity ($v_{Si}$ $\sim$ 13,000 km s$^{-1}$ around the maximum light) than the normal-velocity SNe Ia. We discuss the relation between the $uvw1-v$ color and some observables, including Si~{\sc ii} velocity, line strength of Si~{\sc ii} $\lambda$6355, Fe~{\sc ii}/{\sc iii} lines and $\Delta m_{15}$(B). Compared to other fast-expanding SNe Ia, SN 2013gs exhibits Si and Fe absorption lines with similar strength and bluer $uvw1-v$ color. We briefly discussed the origin of the observed UV dispersion of SNe Ia.