Overview of the 511keV diffuse emission

Abstract

The 511 keV gamma-ray line emission from the inner part of our Galaxy is the signature of annihilation of low energy positrons in this region. This emission has been observed and studied since the seventies, and though several sources have been proposed (type Ia supernovae, low mass X-ray binaries, Sgr A , dark matter...), the origin of Galactic positrons remains an open question. We present the status of our knowledge on this topic in view of recent measurements made with the spectrometer SPI onboard ESA's INTEGRAL observatory. Assuming that positrons do not propagate far from their sources, the morphology of the 511 keV line emission should be tied to the spatial distribution of sources. However, the transport mechanism of positrons in the interstellar medium is still uncertain and recent studies suggest that positrons could propagate more or less far from their source before annihilating, depending on their initial kinetic energy. Implications of measurements and propagation studies on the possible origin of galactic positrons is briefly discussed.

INTEGRAL: Science Highlights and Future Prospects

“…Precision measurements of the line shape and positronium fraction identify the gas properties of the annihilation site (Churazov et al, 2005(Churazov et al, , 2011. This is key information for the determination of the origin of the positrons (Jean et al, 2009), and achievable within the extended INTEGRAL mission. Both, cosmic-ray detectors near Earth, and the Fermi satellite, have measured positrons directly at GeV energies and above.…”