The lowest of the low: discovery of SN 2019gsc and the nature of faint Iax supernovae

Abstract

We present the discovery and optical follow-up of the faintest supernova-like transient known. The event (SN 2019gsc) was discovered in a star-forming host at 53\,Mpc by ATLAS. A detailed multi-colour light curve was gathered with Pan-STARRS1 and follow-up spectroscopy was obtained with the NOT and Gemini-North. The spectra near maximum light show narrow features at low velocities of 3000 to 4000 km s${}^{-1}$, similar to the extremely low luminosity SNe 2010ae and 2008ha, and the light curve displays a similar fast decline (\dmr $0.91\pm 0.10$ mag). SNe 2010ae and 2008ha have been classified as type Iax supernovae, and together the three either make up a distinct physical class of their own or are at the extreme low luminosity end of this diverse supernova population. The bolometric light curve is consistent with a low kinetic energy of explosion ($E_{\mathrm{k}}\sim 10^{49}$ erg s${}^{-1}$), a modest ejected mass ($M_{\mathrm{ej}}\sim 0.2$ \msol) and radioactive powering by ${}^{56}$Ni ($M_{\mathrm{Ni}}\sim 2\times 10^{-3}$ \msol). The spectra are quite well reproduced with radiative transfer models (TARDIS) and a composition dominated by carbon, oxygen, magnesium, silicon and sulphur. Remarkably, all three of these extreme Iax events are in similar low-metallicity star-forming environments. The combination of the observational constraints for all three may be best explained by deflagrations of near $M_{\mathrm{Ch}}$ hybrid carbon-oxygen-neon white dwarfs which have short evolutionary pathways to formation.

12 pages, 5 figures, accepted to ApJL, minor changes to submitted version