Fresh look at the diffuse ALP background from supernovae

Protoneutron stars, highly compact objects formed in the core of exploding supernovae (SNe), are powerful sources of axionlike particles (ALPs). In the SN core, ALPs are dominantly produced via nucleon-nucleon bremsstrahlung and pion conversion, resulting in an energetic ALP spectrum peaked at energies O(100) MeV. In this work, we revisit the diffuse ALP background, produced from all past corecollapse supernovae, and update the constraints derived from Fermi-LAT observations. Assuming the maximum ALP-nucleon coupling allowed by the SN 1987A cooling, we set the upper limit ga gamma gamma 2 x 10-13 GeV-1 for ALP mass ma 10-10 eV, which is approximately a factor of two improvement with respect to the existing bounds. On the other hand, for ma greater than or similar to 10-10 eV, we find that including pion conversion strengthens the bound on ga gamma gamma, approximately by a factor of two compared to the constraint obtained from bremsstrahlung alone. Additionally, we present a sensitivity study for future experiments such as AMEGO-X, e-ASTROGAM, GRAMS-balloon, GRAMS-satellite, and MAST. We find that the expected constraint from MAST would be comparable to Fermi-LAT bound. However, SN 1987A constraint remains one order of magnitude stronger as compared to the bound derived from the current and future gamma-ray telescopes.