Invited Talk - Plenary

Tuesday, 13 September 2022, 11:00   (Keksdose / virtual plenum)

Positron annihilation as an astrophysical messenger

Thomas Siegert
Julius Maximilians Universität Würzburg

The energy band between 0.1 and 10 MeV is the least explored region in astrophysics. Due to the necessity of satellite experiments, huge instrumental background, and abstract data spaces, the sensitivity curves of soft gamma-ray instruments describe the so-called “MeV sensitivity gap”. In this gap, however, the strongest persistent and extended emission line is found, the 511 keV line from electron positron annihilation in the Galactic bulge. The origin of these positrons is unknown — not because the sources would be unknown but rather because there are too many possibilities. Every astrophysical source either creates positrons itself or can show a 511 keV line due to secondary interactions. The supposedly dominant contributors are massive stars and supernovae due to beta-plus-unstable isotopes, black holes and neutron stars via pair-creation, flaring stars, or even dark matter in the form of beyond standard model particles or primordial black holes. Alas, measuring the annihilation emission is showing the sinks of positrons, not their sources, as they propagate through the interstellar medium as low-energy cosmic-ray antiparticles. The 511 keV line describes the natural connection between stars, nucleosynthesis, compact objects, dark matter, cosmic rays and the interstellar medium, and thus has the potential to be used as an astronomical tool. I will present my work on this “positron puzzle”, how problems in astrophysics can be solved by detailed measurements of the annihilation line and MeV sources, and the future of soft gamma-ray instruments.