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

Abstract The nuclear gamma-ray ( $\gamma $ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -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 $ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray line observations provide the most direct evidence for ongoing nucleosynthesis in galaxies by measuring freshly produced radioactive isotopes from massive stars, supernovae (SNe), classical novae (CNe), or binary neutron star mergers (BNSMs). Their flux ratios can determine the low-energy cosmic ray (LECR) spectrum in different objects and of the Milky Way as a whole. Different phases of the interstellar medium (ISM) are traced by hot nucleosynthesis ejecta, cooling positrons, or cosmic ray (CR) 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 (BSM) physics might have their elusive imprints in $\gamma $ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray lines. This article gives an overview of historical $\gamma $ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray line measurements, newest results, and open questions that may only be solved by a new generation of MeV telescopes.