Three-dimensional spatial modelling of nuclear de-excitation lines
Abstract
Galactic cosmic rays (GCRs) are thought to originate from shock acceleration in supernova remnants, but interstellar ionization rates suggest an additional low-energy cosmic-ray (LECR) component. LECRs excite nuclei in the ISM, producing MeV gamma-ray line emission through subsequent de-excitation. Observations of this emission can constrain the LECR flux, morphology, and acceleration mechanisms. We construct all-sky maps of Galactic de-excitation emission using three-dimensional (3D) CR distribution models. By comparing them with the Galactic diffuse gamma-ray emission, we assess the detectability of the de-excitation line components against the underlying continuum. We calculate the emission by combining 3D CR distributions from GALPROP and gamma-ray production cross sections from TALYS. We also model an additional LECR component motivated by the high ionization rates measured in the ISM. The predicted de-excitation emission is relatively weak when integrated over the broad regions commonly adopted in MeV gamma-ray analyses (|l|≤30∘ and |b|≤15∘), contributing only ≲ 1% above the leptonic continuum even considering the additional LECR component. The all-sky maps reveal that, in the standard scenario, the de-excitation-to-leptonic flux ratio remains low over the entire sky, reaching at most ∼20% toward the Galactic anticenter. In contrast, models including an additional LECR component exhibit a pronounced enhancement toward the GC, where the ratio exceeds 100%, indicating favourable prospects for detection through spatially resolved analyses. The 6.1-to-4.4 MeV intensity ratio is particularly sensitive to the maximum energy of the injected LECR spectrum, varying by up to ∼ 30%. Future MeV gamma-ray observatories could thus identify a distinct LECR population and constrain its acceleration and transport.
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BibTeX
@misc{oka2026threedimensional,
title = {{Three-dimensional spatial modelling of nuclear de-excitation lines}},
author = {Tomohiko Oka and Thomas Siegert and Tristan Bouchet and Laura Eisenberger and Mika A. Gelowicz and Saurabh Mittal and Rudi Reinhardt and Manuel R. H. W. Skalka and Dimitris Tsatsis and Hiroki Yoneda and Manja Zimmerer},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.23380},
eprint = {2610.23380},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.23380},
}
RIS
TY - UNPB
TI - Three-dimensional spatial modelling of nuclear de-excitation lines
AU - Tomohiko Oka
AU - Thomas Siegert
AU - Tristan Bouchet
AU - Laura Eisenberger
AU - Mika A. Gelowicz
AU - Saurabh Mittal
AU - Rudi Reinhardt
AU - Manuel R. H. W. Skalka
AU - Dimitris Tsatsis
AU - Hiroki Yoneda
AU - Manja Zimmerer
PY - 2026
DA - 2026/10/16
PB - arXiv
DO - 10.48550/arxiv.2610.23380
UR - https://arxiv.org/abs/2610.23380
AB - Galactic cosmic rays (GCRs) are thought to originate from shock acceleration in supernova remnants, but interstellar ionization rates suggest an additional low-energy cosmic-ray (LECR) component. LECRs excite nuclei in the ISM, producing MeV gamma-ray line emission through subsequent de-excitation. Observations of this emission can constrain the LECR flux, morphology, and acceleration mechanisms. We construct all-sky maps of Galactic de-excitation emission using three-dimensional (3D) CR distribution models. By comparing them with the Galactic diffuse gamma-ray emission, we assess the detectability of the de-excitation line components against the underlying continuum. We calculate the emission by combining 3D CR distributions from GALPROP and gamma-ray production cross sections from TALYS. We also model an additional LECR component motivated by the high ionization rates measured in the ISM. The predicted de-excitation emission is relatively weak when integrated over the broad regions commonly adopted in MeV gamma-ray analyses (|l|≤30^∘ and |b|≤15^∘), contributing only ≲ 1% above the leptonic continuum even considering the additional LECR component. The all-sky maps reveal that, in the standard scenario, the de-excitation-to-leptonic flux ratio remains low over the entire sky, reaching at most ∼20% toward the Galactic anticenter. In contrast, models including an additional LECR component exhibit a pronounced enhancement toward the GC, where the ratio exceeds 100%, indicating favourable prospects for detection through spatially resolved analyses. The 6.1-to-4.4 MeV intensity ratio is particularly sensitive to the maximum energy of the injected LECR spectrum, varying by up to ∼ 30%. Future MeV gamma-ray observatories could thus identify a distinct LECR population and constrain its acceleration and transport.
ER -