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Abstract

Axion-like particles (ALPs) may be abundantly produced in core-collapse (CC)
supernovae (SNe), hence the cumulative signal from all past SN events would
contain an ALP component and create a diffuse flux peaked at energies of about
50 MeV. We update the calculation of this flux by including a set of CC SN
models with different progenitor masses following the expected mass
distribution. Additionally, we include the effects of failed CC SNe, which
yield the formation of black holes instead of explosions. Relying on the
coupling strength of ALPs to photons and the related Primakoff process, the
diffuse SN ALP flux is converted into a diffuse gamma-ray flux while traversing
the magnetic field of the Milky Way. The spatial morphology of this signal is
expected to follow the shape of the Galactic magnetic field lines. We make use
of this via a template-based analysis that utilizes 12 years of {\it Fermi}-LAT
data in the energy range from 50 MeV to 500 GeV. This strategy yields an
improvement of roughly a factor of two for the upper limit on the ALP-photon
coupling constant $g_{a\gamma}$ compared to a previous analysis that accounted
only for the spectral shape of the signal. While the improved SN modeling leads
to a less energetic flux that is harder to detect, the combined effect is still
an improvement of the limit and in particular its statistical reliability. We
also show that our results are robust against variations in the modeling of
high-latitude Galactic diffuse emission and systematic uncertainties of the
LAT.