Weekly seminars

The Galactic 511 keV Positron Puzzle

by Thomas Siegert (Univ. California San Diego)

Auditorium (LAPTh)



9, chemin de Bellevue Annecy-le-Vieux 74000 ANNECY

For half a century, the strongest, persistent, diffuse, gamma-ray line signal from the annihilation of electrons with positrons is puzzling theorists and observers. Unlike at any other wavelength, this 511 keV emission is dominated by a bright bulge emission, in addition to a very low surface-brightness disk. The two main questions of this ‘positron puzzle’ are summarised into “where do the positrons come from?”, and “why does the emission look like that?”. While the problem is not finding a single source to sustain the annihilation rate - it is rather that there are too many possibilities to explain the observations. Most astrophysical phenomena, such as massive stars, supernovae, compact objects, cosmic rays or dark matter can - in principle - produce positrons; however direct observations are difficult. I will present a summary of the ‘positron puzzle’, recent updates on the 511 keV emission morphology and its kinematics with data from INTEGRAL/SPI, as well as an unpopular attempt to balance the positron budget by using Chatton’s antirazor. Finally, I will provide an outlook on what will be possible with future compact Compton telescopes such as the Compton Spectrometer and Imager, COSI.