2018MNRAS.479L..70D

We present nebular-phase spectra of the Type Ia supernova (SN Ia) 2016brx, a member of the 1991bg-like subclass that lies at the faint end of the SN Ia luminosity function. Nebular spectra are available for only three other 1991bg-like SNe, and their Co line centres are all within <=500 km s^–1 of each other. In contrast, the nebular Co line centre of SN 2016brx is blue-shifted by >1500 km s^–1 compared to them and by ≃1200 km s^–1 compared to the rest frame. This is a significant shift relative to the narrow nebular line velocity dispersion of <=2000 km s^–1 of these SNe. The large range of nebular line shifts implies that the ⁵⁶Ni in the ejecta of SN 1991bg-like events is off-centre by ∼1000 km s^–1 rather than universally centrally confined as previously suggested. With the addition of SN 2016brx, the Co nebular line shapes of 1991bg-like objects appear to connect with the brighter SNe Ia that show double-peaked profiles, hinting at a continuous distribution of line profiles among SNe Ia. One class of models to produce both off-centre and bimodal ⁵⁶Ni distributions is collisions of white dwarfs with unequal and equal masses.