The Moon as a Cosmic-Ray Spectrometer: Prospects for MeV Gamma-Ray Observations

The Moon as a Cosmic-Ray Spectrometer: Prospects for MeV Gamma-Ray Observations

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 cont...

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Bibliographic Details
Main Authors: Tatsuki Fujiwara, Ellis R. Owen, Yoshiyuki Inoue, Manel Errando, Kohei Fukuda, Kazuhiro Nakazawa, Hirokazu Odaka, Keigo Okuma, Kentaro Terada, Naomi Tsuji, Yasunobu Uchiyama, Hiroki Yoneda, Ao Zhang
Format: Article
Language:English
Published: IOP Publishing 2025-01-01
Series:The Astrophysical Journal
Subjects:
Gamma-ray astronomy
The Moon
Cosmic rays
Nuclear physics
Gamma-ray lines
Monte Carlo methods
Online Access:https://doi.org/10.3847/1538-4357/add68b
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Summary: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-Large Area Telescope 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 ^28 Si de-excitation enhanced by the lunar surface composition, the e ^+ e ^− annihilation line, and radioactive decay lines from ^22 Na ( τ  ≈ 3.75 yr) and long-lived ^26 Al ( τ  ≈ 1 Myr). These gamma-ray lines are sensitive to CRs with energies ≲1 GeV 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.
ISSN:1538-4357

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