We present nebular spectra ( 153 rest-frame days after peak brightness ) of the Type Ia supernova ( SN Ia ) SN 2019yvq , which had a bright flash of blue and ultraviolet light after exploding , after which it rose in a manner similar to other SNe Ia .
Although SN 2019yvq displayed several other rare characteristics such as persistent high ejecta velocity near peak brightness , it was not especially peculiar and if the early “ excess ” emission were not observed , it would likely be included in cosmological samples .
The excess flux can be explained by several different physical models linked to the details of the progenitor system and explosion mechanism .
While the early-time data alone can not distinguish between these models , each has unique predictions for the optically thin emission at late times .
In our nebular spectra , we detect strong [ \ion Ca2 ] \lambda \lambda 7291 , 7324 and Ca NIR triplet emission , consistent with a double-detonation explosion .
We do not detect H , He , or [ \ion O1 ] emission , predictions for some single-degenerate progenitor systems and violent white dwarf mergers .
The amount of swept-up H or He is < 2.8 \times 10 ^ { -4 } and 2.4 \times 10 ^ { -4 } M _ { \sun } , respectively .
Aside from strong Ca emission , the SN 2019yvq nebular spectrum is similar to those of typical SNe Ia with the same light-curve shape .
The nebular lines are blueshifted relative to their rest-frame wavelengths , which is atypical for a high-velocity SN Ia like SN 2019yvq .
Comparing to theoretical spectra of a sub-Chandrasekhar mass white dwarf undergoing a double-detonation , we find close agreement with a model with a total progenitor mass of 1.15 M _ { \sun } .
The unique properties of SN 2019yvq suggest that thick He-shell double-detonations only account for 1.1 ^ { +2.1 } _ { -1.1 } \% of the total “ normal ” SN Ia rate .
SN 2019yvq is one of the best examples yet that multiple progenitor channels appear necessary to reproduce the full diversity of “ normal ” SNe Ia .