  We present high-cadence optical and ultraviolet light curves of the normal
Type Ia supernova (SN) 2021aefx, which shows an early bump during the first two
days of observation. This bump may be a signature of interaction between the
exploding white dwarf and a nondegenerate binary companion, or it may be
intrinsic to the white dwarf explosion mechanism. In the case of the former,
the short duration of the bump implies a relatively compact main-sequence
companion star, although this conclusion is viewing-angle dependent. Our
best-fit companion-shocking and double-detonation models both overpredict the
UV luminosity during the bump, and existing nickel-shell models do not match
the strength and timescale of the bump. We also present nebular spectra of SN
2021aefx, which do not show the hydrogen or helium emission expected from a
nondegenerate companion, as well as a radio nondetection that rules out all
symbiotic progenitor systems and most accretion disk winds. Our analysis places
strong but conflicting constraints on the progenitor of SN 2021aefx; no current
model can explain all of our observations.
