$\gamma$-Ray Lines -- Signatures of Nucleosynthesis, Cosmic Rays, Positron Annihilation, and Fundamental Physics

TL;DR

This paper reviews the significance of MeV gamma-ray lines as probes for astrophysical processes like nucleosynthesis, cosmic rays, and positron annihilation, highlighting recent findings and future prospects for new telescopes.

Contribution

It provides a comprehensive overview of historical and recent gamma-ray line measurements, emphasizing their role in understanding fundamental astrophysical and particle physics phenomena.

Findings

Gamma-ray lines directly evidence ongoing nucleosynthesis.

Flux ratios reveal cosmic-ray spectra in different environments.

Positron annihilation traces interstellar medium phases.

Abstract

The nuclear $\gamma$-ray lines in the MeV range of the electromagnetic spectrum hold a vast variety of astrophysical, particle-physical, and fundamental physical information that is otherwise extreme difficult to access. MeV $\gamma$-ray line observations provide the most direct evidence for ongoing nucleosynthesis in galaxies by measuring freshly produced radioactive isotopes from massive stars, supernovae, classical novae, or binary neutron star mergers. Their flux ratios can determine the low-energy cosmic-ray spectrum in different objects and of the Milky Way as a whole. Different phases of the interstellar medium are traced by hot nucleosynthesis ejecta, cooling positrons, or cosmic-ray interactions with molecular clouds. Positron annihilation itself can be considered as an astrophysical messenger as their production and destruction in typical space environments is inevitable. Finally, as-of-yet unknown signatures from beyond standard model physics might have their elusive imprints in $\gamma$-ray lines. This Chapter gives an overview of historical $\gamma$-ray line measurements, newest results, and open questions that may only be solved by a new generation of MeV telescopes.