Gamma-Rays from Positron Annihilation

Understand

SPI on INTEGRAL has provided spectra and a map of the sky in the emission from annihilations of positrons in the interstellar medium of our Galaxy.

• From high-resolution spectra we learned that a warm, partially-ionized medium is the site where the observed gamma-rays originate.
• The gamma-ray emission map shows a major puzzle for broader astrophysics topics, as it is dominated by a bright and extended apparently spherical emission region centered in the Galaxy's center.
• Only recently has the disk of the Galaxy been detected with SPI.