The Lowest of the Low: Discovery of SN 2019gsc and the Nature of Faint Iax Supernovae

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 multicolor light curve was gathered with Pan-STARRS1 and follow-up spectroscopy was obtained with the Nordic Optical Telescope and Gemini-North. The spectra near maximum light show narrow features at low velocities of 3000-4000 km s^-1, similar to the extremely low-luminosity SNe 2010ae and 2008ha, and the light curve displays a similar fast decline (Δm₁₅(r) = 0.91 ± 0.10 mag). SNe 2010ae and 2008ha have been classified as SNe Iax, 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_k ∼ 10⁴⁹ erg s^-1), a modest ejected mass (M_ej ∼ 0.2 M_⊙), and radioactive powering by ⁵⁶Ni (M_Ni ∼ 2 × 10^-3 M_⊙). The spectra are quite well reproduced with radiative transfer models (TARDIS) and a composition dominated by carbon, oxygen, magnesium, silicon, and sulfur. 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_Ch hybrid carbon-oxygen-neon white dwarfs that have short evolutionary pathways to formation.