gamma-ray astronomy

The study of gamma-rays from astronomical sources. Gamma rays are the most energetic form of electromagnetic radiation, with wavelengths shorter than X-rays (i.e. less than about 0.1 nanometres). They are absorbed high in the Earth's atmosphere; only the most energetic can be detected at ground level and virtually all astronomical gamma-ray observations must be conducted from satellites. The detectors used are scintillation counters, spark chambers and solid-state detectors, and the angular resolution achieved is coarse by astronomical standards. Detectors for gamma-ray bursters have been carried on numerous spacecraft since 1969. Sky surveys have been carried out by the satellites SAS-2 and COS-B. SAS-2 was launched in 1972 and operated for seven months. COS-B was launched in 1975 and operated for over six years. A great advance in gamma-ray astronomy was achieved with the launch of the Compton Gamma Ray Observatory by NASA in April 1991. Within months of launch, many new sources were being identified with greater positional accuracy than was possible previously. Astronomical sources of gamma rays include solar flares, pulsars, X-ray binary stars and quasars, as well as the gamma-ray bursters. Known discrete sources of gamma-rays include the Vela Pulsar, the Crab Pulsar , SS433 and Geminga. The most intense diffuse gamma radiation comes from the galactic plane, generated by interactions between cosmic rays and the interstellar gas. A gamma-ray spectrometer on HEAO-3 in 1979 observed lines produced by electron-positron annihilation from the direction of the galactic centre.