Galactic Gamma Ray Line Spectroscopy — An Observational Overview

Abstract

Gamma ray line spectroscopy is a powerful tool to study nuclear processes in our Galaxy. Line emission is expected from nuclear reactions of energetic particles, and from nucleosynthesis processes. Possible sources of gamma ray line emission in the Galaxy are discrete objects (like special stars, supernova remnants or interstellar clouds) and the interstellar medium as a whole. During the last decade the first two galactic gamma ray lines have been detected; these are the 511 keV annihilation line, and the 1.8 MeV gamma ray line from radioactive 26Al. A summary of the observational status of the two gamma ray lines is presented, and the origin of both lines is discussed. From this discussion it will become clear that the question of their origin can only be solved by mapping the entire galactic plane in the light of these lines.