Compton scatter imaging in astrophysics

Abstract

Compton telescopes have been an important tool in gamma ray astronomy, most recently with the COMPTEL instrument on NASA's COMPTON Gamma Ray Observatory (CGRO). Scientific objectives in low/medium energy gamma ray astronomy place highest priority on significant improvement in sensitivity relative to CGRO and ESA's planned INTEGRAL mission. The observational requirements include both discrete and extended sources, and narrow, broad, and continuum spectral features. A high spectral and spatial resolution Compton telescope is the preferred instrument for a future mission. The broad range of scientific objectives that such a mission will address include: mapping radioactivity across the Galaxy (/sup 26/Al, /sup 44/Ti, /sup 60/Fe), detection of several Type Ia supernovae per year (/sup 56/Ni, /sup 56/Co), study nuclear burning in novae (/sup 22/Na, /sup 7/Be, 511 keV), study the accretion of matter onto galactic and massive extra-galactic black holes and clarification of the mechanism for the formation of relativistic jets; provide unique insights into the structure of neutron stars, cosmic gamma ray bursts; and solar flares. A mission to achieve these broad goals should have narrow line sensitivities approaching 10/sup -7/ /spl gamma//cm/sup 2/-s for discrete sources. An energy threshold of /spl sim/250 keV is required to ensure good coverage of the many important astrophysical gamma ray lines from 511 keV to 2 MeV. Angular resolutions much better than 1 degree are required. Detectors which combine state of the art energy resolution (few keV) and spatial resolution (1 mm) are needed. A major breakthrough in background reduction and sensitivity could be realized if tracking low-energy electrons (/spl sim/100 keV) can be achieved.