2025/04/09 by Fujiwara, Tatsuki, Owen, Ellis R., Inoue, Yoshiyuki +10
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2504.07195
The Moon is the closest celestial gamma-ray emitting object. Its gamma-ray emission arises from interactions between Galactic cosmic rays (CRs) and the lunar surface. While the lunar GeV gamma-ray spectrum is dominated by a continuum from hadronic decay processes, the MeV emission exhibits both continuum and distinctive spectral lines from nuclear de-excitation and radioactive decay processes. Using Geant4 Monte Carlo particle simulations, we model the lunar gamma-ray spectrum. Our results demonstrate its consistency with Fermi-LAT observations, and predict that next-generation MeV gamma-ray instruments will detect both the lunar MeV continuum and several key spectral line features, notably the 1.779~MeV line from \mathrm28Si de-excitation enhanced by the lunar surface composition, the e+e- annihilation line, and radioactive decay lines from \mathrm22Na (τ≈3.75 yr) and long-lived \mathrm26Al (τ≈1 Myr). These gamma-ray lines are sensitive to CRs with energies \lesssim1 \mathrmGeV nuc-1, offering unique temporal probes of CR activity over different timescales. Observations of the lunar MeV gamma-ray spectrum will therefore open a new window to study the current irradiation of the solar-terrestrial environment by low-energy CRs and its long-term temporal evolution.