Variable H$\alpha$ 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 $\sim
65$~kpc from the S0 galaxy IC~550 at $z=0.0165$ ($D\approx 74$~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
$s_{BV} \approx 0.6$ and $B$-band decline rate $\Delta m_{15}(B) \approx 1.77$
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 H$\alpha$ line in
emission that is resolved in the first spectrum with $\rm FWHM \approx 1200$ km
s$^{-1}$ and $L_{H\alpha} \approx 3.8\times 10^{37}$ erg s$^{-1}$. The
detection of a resolved H$\alpha$ line with a declining luminosity is broadly
consistent with recent models where hydrogen is stripped from the
non-degenerate companion in a single-degenerate progenitor system. However, the
amount of hydrogen consistent with the luminosities of the H$\alpha$ line would
be $\sim 10^{-3}$ M$_{\odot}$, significantly less than theoretical model
predictions in the classical single-degenerate progenitor systems.
SN2018cqj/ATLAS18qtd is the second low-luminosity, fast-declining Type Ia SN
after SN2018fhw/ASASSN-18tb that shows narrow H$\alpha$ in emission in its
nebular-phase spectra.
