Observations of a Fast-expanding and UV-bright Type Ia Supernova SN 2013gs

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 are similar to those of normal SNe Ia, with an absolute peak magnitude of M <SUB> B </SUB> = -19.25 ± 0.15 mag and a post-maximum decline rate ∆m <SUB>15</SUB>(B) = 1.00 ± 0.05 mag. Gehrels Swift Ultra-Violet/Optical Telescope observations indicate that SN 2013gs shows unusually strong UV emission (especially in the uvw1 band) at around the maximum light (M <SUB> uvw1</SUB> ∼ -18.9 mag). The SN is characterized by relatively weak Fe II III absorptions at ∼5000 Å in the early spectra and a larger expansion velocity (v <SUB>Si</SUB> ∼ 13,000 km s<SUP>-1</SUP> around the maximum light) than the normal-velocity SNe Ia. We discuss the relation between the {uvw}1-v color and some observables, including Si II velocity, line strength of Si II λ6355 and Fe II/III lines, and ∆m <SUB>15</SUB>(B). Compared to other fast-expanding SNe Ia, SN 2013gs exhibits Si and Fe absorption lines with similar strength and bluer {uvw}1-v color. We briefly discussed the origin of the observed UV dispersion of SNe Ia.