Gamma-Ray Astrophysics

γ-ray astronomy is the study of the most energetic photons originating in our Galaxy and beyond, and therefore, provides the most direct means of studying the largest transfers of energy occurring in astrophysical processes. The first certain detection of celestialγ-rays came from a satellite experiment flown on OSO-III (Kraushaaret al., 1972); more recently two second generation spark chamberγ-ray telescopes, flown on the SAS-2 (Fichtelet al., 1975) and COS-B (Bennettet al., 1974) satellites, are now obtaining more detailed results on the high energy celestial radiation causingγ-ray astronomy to move from the discovery phase to the exploratory phase. The most striking feature of the celestial sphere when viewed in the frequency range ofγ-rays is the emission from the galactic plane, which is particularly intense in the galactic longitudinal region from 300° to 50°. The longitudinal and latitudinal distributions are generally correlated with galactic structural features and when studied in detail suggest a non-uniform distribution of cosmic rays in the galaxy. Several pointγ-ray sources have now been observed, including four radio pulsars. This last result is particularly striking since only one radio pulsar has been seen at either optical or X-ray frequencies. Nuclearγ-ray lines have been seen from the Sun during a large solar flare and future satellite experiments are planned to search forγ-ray lines from supernovae and their remnants. A general apparently diffuse flux ofγ-rays has also been seen whose energy spectrum has interesting implications; however, in view of the possible contribution of point sources and the observation of galactic features such as Gould's belt, its interpretation must awaitγ-ray experiments with finer spatial and energy resolution, as well as greater sensitivity. Instruments with much greater sensitivity and improved energy and angular resolution are now available and will greatly enhance our understanding of high energy processes in astrophysics, especially in view of the high penetrating power ofγ-rays, which for example permit them to reach the solar system from the far side of the galaxy essentially unattenuated.