Abstract
Abstract We present the discovery of ZTF 21aaoryiz/SN 2021fcg—an extremely low luminosity Type Iax supernova. SN 2021fcg was discovered by the Zwicky Transient Facility in the star-forming galaxy IC0512 at a distance of ≈27 Mpc. It reached a peak absolute magnitude of M r = −12.66 ± 0.20 mag, making it the least luminous thermonuclear supernova discovered to date. The E(B − V) contribution from the underlying host galaxy is unconstrained. However, even if it were as large as 0.5 mag, the peak absolute magnitude would be M r = −13.78 ± 0.20 mag—still consistent with being the lowest-luminosity SN. Optical spectra of SN 2021fcg taken at 37 and 65 days post-maximum show strong [Ca ii], Ca ii, and Na i D emission and several weak [Fe ii] emission lines. The [Ca ii] emission in the two spectra has extremely low velocities of ≈1300 and 1000 km s−1, respectively. The spectra very closely resemble those of the very low luminosity Type Iax supernovae SN 2008 ha, SN 2010ae, and SN 2019gsc taken at similar phases. The peak bolometric luminosity of SN 2021fcg is ≈ 2.5 − 0.3 + 1.5 × 10 40 erg s−1, which is a factor of 3 lower than that for SN 2008 ha. The bolometric lightcurve of SN 2021fcg is consistent with a very low ejected nickel mass (M Ni ≈ 0.8 − 0.5 + 0.4 × 10 − 3 M ⊙). 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-M ch hybrid carbon–oxygen–neon white dwarfs. Instead, the merger of a carbon–oxygen and oxygen–neon white dwarf is a promising model to explain SN 2021fcg.
Highlights
Type Iax supernovae (SNe) are a peculiar subclass of Type Ia SNe (Foley et al 2013)
In this Letter, we present the discovery of Zwicky Transient Facility (ZTF) 21aaoryiz or SN 2021fcg—the least luminous member of the SN Iax class
SN 2021fcg was discovered by the Zwicky Transient Facility (ZTF; Bellm et al 2019; Graham et al 2019; Dekany et al 2020), which runs on the Palomar 48 inch (P48) Oschin Schmidt telescope
Summary
Type Iax supernovae (SNe) are a peculiar subclass of Type Ia SNe (Foley et al 2013). These events are named after the prototypical SN 2002cx (Li et al 2003) and are characterized by slower expansion speeds (2000–8000 km s−1) and a diverse range of luminosities compared to normal SNe Ia (Jha 2017). Four SNe Iax have been discovered with very low luminosities (MV ≈ −14) and explosion energies—SN 2008 ha (MV = −14.2; Foley et al 2009; Valenti et al 2009), SN 2010ae (−13.8 > MV > −15.3; Stritzinger et al 2014), SN 2019gsc (Mr = −13.9; Srivastav et al 2020; Tomasella et al 2020), and SN 2019ttf (Mr ≈ −14; De et al 2020), only the first three have been studied extensively These SNe have low ejected nickel masses (∼10−3 Me) and a faster evolution than their brighter counterparts.
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