Positron Annihilation in the Galaxy

Understand

The 511 keV line from positron annihilation in the Galaxy was the first $\gamma$-ray line detected to originate from outside our solar system.

• Going into the fifth decade since the discovery, the source of positrons is still unconfirmed and remains one of the enduring mysteries in $\gamma$-ray astronomy.
• With a large flux of $\sim$10$^{-3}$ $\gamma$/cm$^{2}$/s, after 15 years in operation INTEGRAL/SPI has detected the 511 keV line at $>50\sigma$ and has performed high-resolution spectral studies which conclude that Galactic positrons predominantly annihilate at low energies in warm phases of the interstellar medium.
• The results from imaging are less certain, but show a spatial distribution with a strong concentration in the center of the Galaxy.