Electron-Positron Annihilation in Our Galaxy

Dear Colleagues,

The 511 keV gamma-ray emission from our Galaxy is an intriguing and still unsolved problem in Astrophysics. We know that a huge number of positrons (about 10 43 ) annihilated every second in the Galaxy, but we do not know exactly where they come from, even though many candidates have been proposed.

The first evidence of a 511 keV diffuse emission from the center of the Galaxy dates back to the early seventies. The most recent data has been obtained thanks to the observations made using the high resolution spectrometer SPI, and the gamma-ray imager IBIS, both on board the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite. The SPI data provides a detailed all-sky map of the 511 keV emission with a very high spectral resolution (1). The IBIS data has been used to obtain robust upper limits on the 511 keV emission from known Galactic objects such as X-ray binaries or supernovae (2).

In this Special Issue, we aim to collect both review and original papers on the 511 keV line emission in our Galaxy. Contributions based on new analysis of the available data are welcome. Contributions related with the applications of new analysis tools, and development of new instruments are encouraged. Papers on new physical models and ideas to explain observations are also appreciated.

Some of the key topics that will be covered in this Special Issue of Galaxies are:

• All sky map of the annihilation diffuse emission.
• High resolution spectral analysis of the 511 keV line and positronium continuum.
• Search for 511 keV point sources.
• Map of ²⁶ Al 1.8 MeV and ⁴⁴ Ti 1.157 MeV diffuse emission along the Galactic plane to estimate the contribution due to supernovae.
• Constraints on positrons injection energy by measuring the in-flight annihilation spectrum.
• Study of the propagation of the positrons in the interstellar medium to answer how far the positrons travel before annihilation.
• Observational and theoretical studies on the possible sources of Galactic positrons
• Design and development of new missions

References:

1. Siegert, T., Diehl, R., Khachatryan, G., et al. 2016, A&A, 586, A84
2. De Cesare, G. 2011, A&A, 531, A56

Dr. Giovanni De Cesare
Guest Editor

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• gamma-ray-astronomy
• annihilation