The 511 keV emission from positron annihilation in the Galaxy

Understand

The first gamma-ray line originating from outside the solar system that was ever detected is the 511 keV emission from positron annihilation in the Galaxy.

• Despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now.
• Observations in the 1990's with OSSE/CGRO showed that the emission is strongly concentrated towards the Galactic bulge.
• In the 2000's, the SPI instrument aboard ESA's INTEGRAL gamma-ray observatory allowed scientists to measure that emission across the entire Galaxy, revealing that the bulge/disk luminosity ratio is larger than observed in any other wavelength.