We present new 0.3-21 micron photometry of SN 2021aefx in the spiral galaxy
NGC 1566 at +357 days after B-band maximum, including the first detection of
any SN Ia at >15 micron. These observations follow earlier JWST observations of
SN 2021aefx at +255 days after the time of maximum brightness, allowing us to
probe the temporal evolution of the emission properties. We measure the
fraction of flux emerging at different wavelengths and its temporal evolution.
Additionally, the integrated 0.3-14 micron decay rate of Δm0.3−14=1.35±0.05 mag/100 days is higher than the decline rate from the
radioactive decay of 56Co of ∼1.2mag/100 days. The most plausible
explanation for this discrepancy is that flux is shifting to >14 micron, and
future JWST observations of SNe Ia will be able to directly test this
hypothesis. However, models predicting non-radiative energy loss cannot be
excluded with the present data.Comment: Accepted for publication in ApJL; 11 pages, 4 figures, 2 tables in
two-column AASTEX63 forma