[Paper Review] Positron Annihilation in the Galaxy

Nikos Prantzos|HAL (Le Centre pour la Communication Scientifique Directe)|Sep 10, 2019

Atomic and Molecular Physics4 citations

TL;DR

This white paper advocates for a next-generation, all-sky, direct-imaging gamma-ray telescope with sub-degree angular resolution, sub-1% energy resolution, and 100× improved sensitivity to the 511 keV positron annihilation line. It proposes that such a mission would resolve the origin of Galactic positrons by mapping emission across the sky, measuring in-flight annihilation spectra, and constraining propagation and source models—including nucleosynthesis, compact objects, and dark matter—thereby resolving one of the longest-standing puzzles in high-energy astrophysics.

ABSTRACT

International audience

Motivation & Objective

• To resolve the long-standing mystery of the origin of Galactic positrons, which remain unexplained despite 50 years of observation.
• To enable direct imaging of the 511 keV line and positronium continuum across the entire sky with high significance.
• To measure the in-flight annihilation spectrum above 511 keV to constrain positron injection energy and source models.
• To improve constraints on positron propagation in the interstellar medium by comparing 511 keV and 26Al/44Ti spatial distributions.
• To test competing models including millisecond pulsars, supernovae, neutron star mergers, and dark matter via high-sensitivity, high-resolution gamma-ray observations.

Proposed method

• Deploy a wide-field ( >2 sr), direct-imaging gamma-ray telescope with ~1° angular resolution and ~1% FWHM energy resolution over 200 keV to few MeV.
• Conduct an all-sky survey mode to achieve ~10⁻⁶ γ/cm²/s sensitivity for narrow lines, enabling detection of low-surface-brightness emission.
• Use Compton imaging techniques to constrain the spatial extent and morphology of the 511 keV emission, distinguishing between diffuse and halo components.
• Measure the ratio of the 511 keV line to the positronium continuum to infer annihilation conditions and energy loss in the ISM.
• Map the 1.8 MeV 26Al and 1.157 MeV 44Ti emissions in parallel with 511 keV to compare source contributions and propagation distances.
• Utilize improved continuum sensitivity between 511 keV and a few MeV to detect the in-flight annihilation spectrum and infer initial positron energy.

Experimental results

Research questions

• RQ1 What is the true spatial distribution of 511 keV emission across the Milky Way, and does it exhibit a diffuse or structured morphology?
• RQ2 What is the energy spectrum of positrons at injection, as revealed by the in-flight annihilation continuum above 511 keV?
• RQ3 How do positron propagation distances and energy loss vary across different regions of the interstellar medium, particularly in the Galactic bulge and disk?
• RQ4 To what extent do nucleosynthetic sources (26Al, 44Ti) and compact object populations (e.g., millisecond pulsars) contribute to the observed 511 keV emission?
• RQ5 Can the 511 keV emission be used to constrain the mass and interaction cross-section of light dark matter particles?

Key findings

• The 511 keV line is the strongest persistent diffuse gamma-ray line in the Galaxy, with a flux of ~10⁻³ γ/cm²/s toward the Galactic center.
• Over 95% of positrons annihilate at low energies (~eV), indicating rapid thermalization in the interstellar medium, primarily in warm ionized and neutral phases.
• The positronium continuum is precisely measured by SPI, confirming that most annihilation occurs via charge exchange with interstellar gas.
• A halo component of 511 keV emission may exist, which would significantly alter the total annihilation rate and require improved sensitivity to detect.
• The 1.8 MeV 26Al line has been detected in regions like Cygnus, and a corresponding 511 keV flux of ~6×10⁻⁵ γ/cm²/s is expected if positrons annihilate quickly after injection.
• Improved sensitivity to the 1.157 MeV 44Ti line would allow direct comparison with 511 keV emission without relying on uncertain supernova yields.