The study of electromagnetic radiation from space at the very shortest wavelengths and with the highest photon energies (see gamma rays). Gamma rays are produced in regions of extremely high temperature, density, and magnetic fields, sites of the most violent processes in the Universe.
Many hundreds of individual gamma-ray sources are known, as well as a general gamma-ray background. Early experiments in the 1950s and 1960s used balloons to carry instruments to altitudes where the atmospheric absorption of gamma rays is low. Exploratory observations were also made with spacecraft, including Ranger and Apollo missions, during the 1960s. The first sky surveys were made by the satellites SAS-2 (see small astronomy satellite) and COS-B, launched in 1972 and 1974. In the late 1970s two High Energy Astrophysical Observatories (HEAO-1 and HEAO-3) carried gamma-ray experiments. The Granat satellite was launched in 1990, the Compton Gamma Ray Observatory in 1991, the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) in 2002, Swift in 2004, and the Fermi Gamma-ray Space Telescope in 2008.
The large energy range involved in gamma-ray astronomy necessitates several observational techniques. Only the very highest energies (above 100 GeV) can penetrate the Earth's atmosphere, so most observations must be made from space. At the lowest energies (100 keV to 10 MeV) gamma-ray telescopes create images using the principle of the Compton effect, collimation, or the coded mask. Between 20 MeV and 30 GeV gamma-ray detection relies on the production of electron pairs using spark chambers and NaI detectors. Above 100 GeV the low photon fluxes require larger instruments than can be carried on satellites. For these energies, the Earth's atmosphere is used as the detector, and optical telescopes record the Cerenkov radiation from the secondary electrons produced by the primary gamma-ray photons.
Subjects: Astronomy and Astrophysics.