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Early observations of Type Ia supernovae (SNe$\,$Ia) provide essential cluesfor understanding the progenitor system that gave rise to the terminalthermonuclear explosion. We present exquisite observations of SN$\,$2019yvq,the second observed SN$\,$Ia, after iPTF$\,$14atg, to display an early flash ofemission in the ultraviolet (UV) and optical. Our analysis finds thatSN$\,$2019yvq was unusual, even when ignoring the initial flash, in that it wasmoderately underluminous for an SN$\,$Ia ($M_g \approx -18.5\,$mag at peak) yetfeatured very high absorption velocities ($v \approx15,000\,\mathrm{km\,s}^{-1}$ for Si II $\lambda$6355 at peak). We find thatmany of the observational features of SN$\,$2019yvq, aside from the flash, canbe explained if the explosive yield of radioactive $^{56}\mathrm{Ni}$ isrelatively low (we measure $M_{^{56}\mathrm{Ni}} = 0.31 \pm 0.05\,M_\odot$) andit and other iron-group elements are concentrated in the innermost layers ofthe ejecta. To explain both the UV/optical flash and peak properties ofSN$\,$2019yvq we consider four different models: interaction between the SNejecta and a nondegenerate companion, extended clumps of $^{56}\mathrm{Ni}$ inthe outer ejecta, a double-detonation explosion, and the violent merger of twowhite dwarfs. Each of these models has shortcomings when compared to theobservations; it is clear additional tuning is required to better matchSN$\,$2019yvq. In closing, we predict that the nebular spectra of SN$\,$2019yvqwill feature either H or He emission, if the ejecta collided with a companion,strong [Ca II] emission, if it was a double detonation, or narrow [O I]emission, if it was due to a violent merger.

The Spectacular Ultraviolet Flash From the Peculiar Type Ia Supernova 2019yvq