New insights from cosmic gamma rays

TL;DR

This paper reviews recent advances in cosmic gamma-ray measurements, highlighting their role in nuclear astrophysics, supernova studies, positron annihilation, and the prospects for future gamma-ray astronomy.

Contribution

It provides a comprehensive overview of recent gamma-ray observations related to nuclear processes in cosmic sources and discusses new insights into positron sources and supernova nucleosynthesis.

Findings

Measurement of radioactive isotopes in supernovae enhances understanding of stellar explosions.

Gamma-ray spectroscopy reveals feedback processes of massive stars into interstellar medium.

Positron annihilation signals suggest microquasars may be significant positron sources.

Abstract

The measurement of gamma rays from cosmic sources at MeV energies is one of the key tools for nuclear astrophysics, in its study of nuclear reactions and their impacts on objects and phenomena throughout the universe. Gamma rays trace nuclear processes most directly, as they originate from nuclear transitions following radioactive decays or high-energy collisions with excitation of nuclei. Additionally, the unique gamma-ray signature from the annihilation of positrons falls into this astronomical window and is discussed here: Cosmic positrons are often produced from beta-decays, thus also of nuclear physics origins. The nuclear reactions leading to radioactive isotopes occur inside stars and stellar explosions, which therefore constitute the main objects of such studies. In recent years, both thermonuclear and core-collapse supernova radioactivities have been measured, and complement conventional supernova observations with measurements of their prime energy sources. The diffuse radioactive afterglow of massive-star nucleosynthesis in gamma rays is now being analysed towards how massive stars feed back their energy and ejecta into interstellar gas. Large interstellar cavities and superbubbles have been recognised to be the dominating structures where new massive-star ejecta are injected, from 26Al gamma-ray spectroscopy. Finally, the puzzling bulge-dominated intensity distribution of positron annihilation gamma rays is measured in greater detail, but still not understood; a recent microquasar flare provided evidence that such objects may be prime sources for positrons in interstellar space, rather than distributed nucleosynthesis. We also briefly discuss the status and prospects for astronomy with telescopes for the nuclear-radiation energy window.