gamma-ray astronomy

gamma-ray astronomy study of astronomical objects by analysis of the most energetic electromagnetic radiation they emit. Gamma rays are shorter in wavelength and hence more energetic than X rays (see gamma radiation ) but much harder to detect and to pinpoint. X rays and some gamma rays are produced throughout the universe by the same catastrophic astrophysical events, such as supernovas and black holes , and gamma-ray astronomy can be considered an extension of X-ray astronomy to the extreme shortwave end of the spectrum .

Gamma rays are difficult to observe from ground-based telescopes due to atmospheric interference, and high-altitude balloons, sounding rockets , and orbiting observatories are therefore used. Some ground-based facilities, including a large 33-ft (10-m) dish with many small mirrors at Mount Hopkins, Ariz., are successful gamma-ray collectors because they record the radiation emitted by very-high-energy gamma rays as they generate high-speed electrons in the upper atmosphere. Another approach to detecting this radiation is the Milagro detector in the Jemez Mountains of New Mexico. It consists of hundreds of phototubes floating within a pond containing 6 million gallons of water; through interactions with the water, the radiation generates weak trails of light that are detected by the phototubes, yielding data about the energy and direction of the gamma rays.

Cygnus X-3 and the Crab and Vela pulsars are well known gamma-ray sources. In addition, gamma rays have been detected as general background radiation concentrated along the plane of the Milky Way. These gamma rays may result from cosmic rays interacting with gaseous matter in the interstellar medium. Gamma rays from outside the Milky Way have been found emanating from radio galaxies (galaxies whose radio emissions constitute an extraordinarily large amount of their total energy output), Seyfert galaxies (galaxies with extremely bright cores—called Active Galactic Nuclei [AGN]—that are strong emitters of radio waves, X rays, and gamma rays), and supernovas.

The first gamma-ray telescope was carried into orbit on the Explorer XI satellite in 1961. Additional gamma-ray experiments flew on the OGO, Vela, and Russian Cosmos series of satellites. The Orbiting Solar Observatory OSO-3 made the first certain detection of celestial gamma rays in 1972, and OSO-7 detected gamma-ray emission lines in the solar spectrum. However, the first satellite designed as a "dedicated" gamma-ray mission was the second Small Astronomy Satellite ( SAS-2 ) in 1972. In 1975 the European Space Agency launched the COS-B satellite to survey the sky for gamma-ray sources. SAS-2 and COS-B confirmed the earlier findings of gamma-ray background radiation and also detected a number of point sources, but the poor resolution of the instruments made it impossible to associate most of these point sources with individual stars or stellar systems. The third High Energy Astronomy Observatory ( HEAO-3 ), launched in 1979, studied both cosmic rays and gamma radiation. A number of satellites launched during the 1980s carried gamma-ray experiments into orbit. The Compton Gamma-Ray Observatory (CGRO), launched in 1991, carried a collection of four instruments that were larger and more sensitive than any gamma-ray telescope previously orbited. In addition to creating a comprehensive map of celestial gamma-ray sources and demonstrating that gamma-ray bursts are evenly distributed across the sky (which suggests that the radiation is coming from the distant reaches of the universe and not just from within the Milky Way), CGRO detected a number of "firsts," such as the first gamma-ray quasar . During the 1990s a number of planetary probes, such as Mars Observer (1983), and earth-orbiting satellites, such as Minisat 1 (1997), carried gamma-ray detection and measurement devices as part of their instrumentation.

The turn of the century saw designs for gamma-ray astronomy satellites that allow for imaging resolution and spectral resolution powers never before possible. Launchings of orbiting gamma-ray observatories include missions such as the High Energy Transient Explorer (HETE-2), launched in 2000, the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, and the Swift Gamma Ray Burst Explorer, launched in 2004.

In 1967 a Vela military satellite designed to detect nuclear explosions discovered the first gamma-ray bursts (GRBs). These events are very short-lived, lasting from about 50 milliseconds to, in extreme cases, several minutes, and occur on an almost daily basis. It has been suggested that the formation of black holes is associated with these intense gamma-ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting—from a few days to several years— electromagnetic radiation (called the afterglow) in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance.

Bibliography: See G. E. Morfill, ed., Galactic Astrophysics and Gamma-Ray Astronomy (1983); P. Murthy and A. Wolfendale, Gamma-Ray Astronomy (1993); N. Gehrels, Gamma Ray Astronomy (1995); T. Weekes, Very High Energy Gamma Ray Astronomy (2003).

gamma-ray astronomy 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, and Swift in 2004.

 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.