Presentation #317.07 in the session Type Ia and Iax Supernovae.
We present the discovery of ZTF 21aaoryiz/SN 2021fcg—an extremely low luminosity Type Iax supernova. SN 2021fcg reached a peak absolute magnitude of Mr ~ -12.6 mag, making it the least luminous thermonuclear supernova discovered to date. Late-time optical spectra of SN 2021fcg show strong [Ca II], Ca II emission with extremely low velocities of ~ 1000 km/s. The spectra closely resemble those of the very low luminosity Type Iax supernovae SN 2008 ha, SN 2010ae, and SN 2019gsc. The peak bolometric luminosity of SN 2021fcg is ~2.5×1040 erg/s, which is a factor of 3 lower than that for the three comparison supernovae. The bolometric lightcurve of SN 2021fcg is consistent with a very low ejected nickel mass (M ~ 0.0008 Msun). The low luminosity and nickel mass of SN 2021fcg pose a challenge to the picture that low-luminosity SNe Iax originate from deflagrations of near-Mch hybrid white dwarfs. Instead, the merger of a carbon–oxygen and oxygen–neon white dwarf is a promising model to explain SN 2021fcg.