A significantly off-centre 56Ni distribution for the low-luminosity type Ia supernova SN 2016brx from the 100IAS survey

Abstract

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.