  We perform general relativistic one-dimensional supernova (SN) simulations to
identify observable signatures of enhanced axion emission from the pion-induced
reaction $\pi^- + p \rightarrow n + a$ inside a newly born protoneutron star
(PNS).We focus on the early evolution after the onset of the supernova
explosion to predict the temporal and spectral features of the neutrino and
axion emission during the first 10 s. Pions are included as explicit new
degrees of freedom in hot and dense matter. Their thermal population and their
role in axion production are both determined consistently to include effects
due to their interactions with nucleons. For a wide range of ambient conditions
encountered inside a PNS, we find that the pion-induced axion production
dominates over nucleon-nucleon bremsstrahlung processes. By consistently
including the role of pions on the dense matter equation of state and on the
energy loss, our simulations predict robust discernible features of neutrino
and axion emission from a galactic supernova that can be observed in
terrestrial detectors. For axion couplings that are compatible with current
bounds, we find a significant suppression with time of the neutrino luminosity
during the first 10 s. This suggests that current bounds derived from the
neutrino signal from SN 1987A can be improved and that future galactic
supernovae may provide significantly more stringent constraints.
