Source location capability of present and future high energy gamma ray telescopes

Abstract

Astrophysical studies with high energy γ-ray telescopes will benefit significantly from improved capability of such telescopes to determine source locations. Instruments under construction use either direct measurement of γ-ray arrival directions (the EGRET instrument on the Gamma Ray Observatory) or coded aperture techniques (the Soviet-French GAMNA-I detector). A comparison of these two approaches shows that either can produce source locations of 5 to 10 arcmin, significantly better than previous results. The conventional telescope achieves this result by using the relatively small number of high energy, high resolution γ-rays, while the coded aperture technique requires high statistics but can use γ-rays of all energies. The coded aperture technique, however, is limited to only the brightest sources. Suggested future direct and coded aperture telescopes can yield better source locations. Conventional high energy γ-ray telescopes offer advantages over coded aperture systems for three reasons: the low statistics inherent in realistic astrophysical observations at these energies limit the operation of the coded aperture detectors; the conventional telescopes gain from the improvement of single photon angular resolution at higher energies; and the cosmic-ray-induced background is a potentially serious problem for coded aperture systems.