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

Thomas Siegert, Francesca Calore, Pierre Jean, Mark Leising, Nicolas de Séréville, Gerald H. Share, Vincent Tatischeff, Wei Wang, Meng-Ru Wu

TL;DR

This chapter surveys the rich science of MeV γ-ray lines as probes of ongoing nucleosynthesis, cosmic-ray interactions, positron annihilation, and potential beyond-Standard-Model physics. It synthesizes historical measurements (e.g., 26Al at 1809 keV, 60Fe lines), current results from instruments like COMPTEL and INTEGRAL/SPI, and detailed modeling of line formation, transport, and opacity. The text highlights cornerstone sources (massive stars, ccSN, SNIa, CN, r-process events) and outlines how line fluxes, Doppler shifts, and line-widths constrain nucleosynthesis yields, Galactic chemical evolution, and explosion physics, while also exploring LECRs, solar-system albedos, and the 511 keV positron line. It emphasizes the uncertainties in nuclear reaction rates, stellar evolution, and positron transport, and argues that a next-generation MeV γ-ray mission is essential to resolve key questions about Galactic nucleosynthesis, the MeV background, and possible DM signatures. Overall, the work champions a coordinated program of laboratory measurements and broad-sky MeV observations to unlock a century-by-century history of element formation in the Universe.

Abstract

The nuclear $γ$-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 $γ$-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 $γ$-ray lines. This Chapter gives an overview of historical $γ$-ray line measurements, newest results, and open questions that may only be solved by a new generation of MeV telescopes.

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

TL;DR

This chapter surveys the rich science of MeV γ-ray lines as probes of ongoing nucleosynthesis, cosmic-ray interactions, positron annihilation, and potential beyond-Standard-Model physics. It synthesizes historical measurements (e.g., 26Al at 1809 keV, 60Fe lines), current results from instruments like COMPTEL and INTEGRAL/SPI, and detailed modeling of line formation, transport, and opacity. The text highlights cornerstone sources (massive stars, ccSN, SNIa, CN, r-process events) and outlines how line fluxes, Doppler shifts, and line-widths constrain nucleosynthesis yields, Galactic chemical evolution, and explosion physics, while also exploring LECRs, solar-system albedos, and the 511 keV positron line. It emphasizes the uncertainties in nuclear reaction rates, stellar evolution, and positron transport, and argues that a next-generation MeV γ-ray mission is essential to resolve key questions about Galactic nucleosynthesis, the MeV background, and possible DM signatures. Overall, the work champions a coordinated program of laboratory measurements and broad-sky MeV observations to unlock a century-by-century history of element formation in the Universe.

Abstract

The nuclear -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 -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 -ray lines. This Chapter gives an overview of historical -ray line measurements, newest results, and open questions that may only be solved by a new generation of MeV telescopes.
Paper Structure (115 sections, 48 equations, 36 figures, 5 tables)

This paper contains 115 sections, 48 equations, 36 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Defining a $\gamma$-Ray Line
  3. Astrophysical $\gamma$-Ray Lines
  4. Doppler Shift
  5. Doppler Broadening
  6. Taking into Account the Instrument
  7. Astrophysical Line Fluxes and Luminosities
  8. Radioactive Decay
  9. Nuclear Excitation
  10. Positron Annihilation
  11. Dark Matter Decay and Annihilation
  12. Opacity to $\gamma$-rays
  13. Observations and Predictions of $\gamma$-Ray Lines
  14. Diffuse Emission from Radioactive $\mathrm{^{26}Al}$
  15. Measurements of the 1809 keV Line
  16. ...and 100 more sections

Figures (36)

  • Figure 1: Nuclear energy level schemes. Transitions associated with observationally relevant gray emission are marked in green. Left: $\mathrm{^{26}Al}$Endt1990Iliadis2011Pleintinger2020. Right: $\mathrm{^{60}Fe}$Rugel2009Pleintinger2020.
  • Figure 2: Compilation of observational maps (top: COMPTEL Oberlack1996; middle: SPI Bouchet2015, compared to a best-fitting 3D population synthesis model, PSYCO, Pleintinger2020), adopted to match the instrument resolution of $3^\circ$. The minimum intensity is set to $5 \times 10^{-5}\,\mathrm{ph\,cm^{-2}\,s^{-1}\,sr^{-1}}$ to mimic potentially observable structures Siegert2023b.
  • Figure 3: Left: Nuclear production and destruction channels of $\mathrm{^{26}Al}$. Stable isotopes are marked in dark grey Prantzos1996Pleintinger2020. Right: Temperature dependence of element abundances in the NeNaMgAl-cycle in an intermediate-mass AGB star. $\mathrm{^{26}Al}$ is shown as yellow shaded region. The x-axis can also be read as radial information with hotter regions being closer to the stellar centre Lugaro2018Pleintinger2020.
  • Figure 4: Left: Schematic representation of the cosmic cycle of matter with metallicity symbolically increasing from blue to green. The cycle progresses in three fundamental steps over three major scales Pleintinger2020. Right: Spatial distribution of nearby OB associations within 3.5 kpc in the Galactic plane. Dots denote the positions of OB associations from the Gaia catalogue Melnik2017. Spiral arm tangents are shown in thick grey lines Pleintinger2020.
  • Figure 5: SPI spectra (blue data points) around the 1809 keV line for three different cases. Top: $\gamma^2$ Velorum from 1745--1840 keV. The line is not detected but a $2\sigma$ upper limit of $1.7 \times 10^{-5}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ is indicated by the green band. The instrumental background in the same energy range is shown as grey histogram, scaled by a factor of 20 for better comparison. Clearly, the higher the background flux, the larger the error bars per 0.5 keV bin. Middle: Region around the Perseus OB associations. The line flux is $(3.6 \pm 0.4) \times 10^{-4}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, broadened by $(1.13 \pm 0.55)$ keV and shifted by $(0.34 \pm 0.22)$ keV. Bottom: Galactic-wide spectrum. The line flux is $(1.87 \pm 0.03) \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ with a Doppler shift of $(0.37 \pm 0.04)$ keV and an astrophysical line width of $(0.62 \pm 0.32)$ keV which corresponds to a thermal broadening around $80\,\mathrm{km\,s^{-1}}$Pleintinger2020.
  • ...and 31 more figures