Requirements of future gamma-ray missions—Imaging

Cosmic gamma-rays are observed to be emitted on a wide range of angular scale and structure from a diverse population of astronomical objects. For the case of Galactic emission a mixture of discrete spectral lines and continuum radiation is derived from point sources, localised regions, as well as a diffuse band along the entire Galactic plane. In the extragalactic context sources range from the “point-like” active galactic nuclei, for which fine angular resolution studies would be a notable advantage, to the all pervading cosmic diffuse gamma-ray background. Nearby galaxies such as the Large Magellanic Cloud and nearby clusters of galaxies provide targets for which a combination of point source and extended emission studies are required. Solar flares require fine angular gamma-ray imaging within a small field of view whereas the enigmatic gamma-ray bursters pose the challenge of the precise positional location of “random” short lived events which arrive isotropically. In this article we have evaluated the imaging requirements of the next generation of gamma-ray telescopes for the known classes of gamma-ray emitting objects and compared them to the performance characteristics achievable with current instrument technology.