New insights from cosmic gamma rays

Abstract

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 β-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 corecollapse supernova radioactivities have been measured though 56 Ni, 56 Co, and 44 Ti lines, and a beginning has thus been made to complement conventional supernova observations with such measurements of the prime energy sources of supernova light created in their deep interiors. The diffuse radioactive afterglow of massive-star nucleosynthesis in gamma rays is now being exploited towards astrophysical studies on how massive stars feed back their energy and ejecta into interstellar gas, as part of the cosmic cycle of matter through generations of stars enriching the interstellar gas and stars with metals. Large interstellar cavities and superbubbles have been recognised to be the dominating structures where new massive-star ejecta are injected, from 26 Al gamma-ray spectroscopy. Also, constraints on the complex interiors of stars derive from the ratio of 60 Fe/ 26 Al gamma rays. 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.

Steady-state nucleosynthesis throughout the Galaxy

“…From INTEGRAL/SPI 26 Al measurements, a value of 1.9±1.1 SN century −1 has been estimated [37]. Correspondingly scaled down due to the above-discussed foreground sources, this obtains 1.3±0.4 SN century −1 [47] .…”

Photons from Neutrinos: The Gamma-Ray Echo of a Supernova Neutrino Burst

“…Realistically, detection requires the signal to be distinguishable above the relevant backgrounds, mainly the diffuse Galactic background at E γ = 0.511 MeV, which peaks in the direction of the Galactic center (see, e.g., Diehl 2013;Roland 2016;Diehl et al 2021;Frontera et al 2021). We do not consider possible contamination of an echo signal from other nearby sources like gamma-ray bursts, other supernovae, supernova remnants, low-mass X-ray binaries, and others.…”