Variable Hα Emission in the Nebular Spectra of the Low-luminosity Type Ia SN2018cqj/ATLAS18qtd

We present optical photometry and spectroscopy of the Type Ia supernova SN2018cqj/ATLAS18qtd. The supernova exploded in an isolated region at ∼65 kpc from the S0 galaxy IC 550 at z = 0.0165 (D ≈ 74 Mpc) and has a redshift consistent with a physical association to this galaxy. Multicolor photometry show that SN2018cqj/ATLAS18qtd is a low-luminosity ($M_{B_{max}}\approx -17.9$ mag), fast-declining Type Ia, with color stretch s _BV ≈ 0.6 and B-band decline rate Δm ₁₅(B) ≈ 1.77 mag. Two nebular-phase spectra obtained as part of the 100IAS survey at +193 and +307 days after peak show the clear detection of a narrow Hα line in emission that is resolved in the first spectrum with FWHM ≈ 1200 km s⁻¹ and L _Hα ≈ 3.8 × 10³⁷ erg s⁻¹. The detection of a resolved Hα line with a declining luminosity is broadly consistent with recent models where hydrogen is stripped from the nondegenerate companion in a single-degenerate progenitor system. However, the amount of hydrogen consistent with the luminosities of the Hα line would be ∼10⁻³ M _⊙, which is significantly less than theoretical model predictions in the classical single-degenerate progenitor systems. SN2018cqj/ATLAS18qtd is the second low-luminosity, fast-declining SN Ia after SN2018fhw/ASASSN-18tb that shows narrow Hα in emission in its nebular-phase spectra.