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).

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gamma-ray astronomy

Study of astronomical objects and phenomena that emit gamma rays. Gamma-ray telescopes are designed to observe high-energy astrophysical systems, including stellar coronas, white dwarf stars, neutron stars, black holes, supernova remnants, clusters of galaxies, and diffuse gamma-ray background radiation found along the plane of the Milky Way Galaxy. Because Earth's atmosphere blocks most gamma rays, observations are generally conducted by high-altitude balloons or spacecraft. In the 1960s defense satellites designed to detect X rays and gamma rays from clandestine nuclear testing serendipitously discovered enigmatic gamma-ray bursts coming from deep space. In the 1970s Earth-orbiting observatories found a number of gamma-ray point sources, including an exceptionally strong one, dubbed Geminga, that was later identified as a pulsar, the nearest yet detected. The Compton Gamma Ray Observatory, launched in 1991, mapped thousands of celestial gamma-ray sources; it also showed that the mysterious bursts are distributed across the sky, implying that their sources are at the distant reaches of the universe rather than in the Milky Way.

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gamma-ray astronomy [′gam·ə ‚rā ə′strän·ə·mē]

(astronomy)

The study of gamma rays from extraterrestrial sources, especially gamma-ray bursts.

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Gamma-Ray Astronomy 

the branch of observational extraterrestrial astronomy associated with investigations of celestial bodies that emit gamma rays. It originated in April 1961 when the instruments in the American artificial satellite Explorer 11 registered gamma radiation emanating from the center of our galaxy. Gamma-ray astronomy is directly related to X-ray astronomy, and the boundary between them is highly arbitrary. It is generally customary to include in gamma-ray astronomy investigations in the spectral region, where the energy of the quanta exceeds 30 keV (corresponding to a wavelength of less than 0.3 angstroms). The earth’s atmosphere is completely opaque for such radiation up to a height of 30-40 km. Consequently, the instruments used in observing gamma rays from celestial bodies (gamma telescopes) are placed, as a rule, on artificial earth satellites, while high-altitude balloons, which can lift the instruments up to 40 km, are used for investigations of hard radiation with energies in the vicinity of 100 keV. The streams of gamma rays observed are very small and therefore require many hours of observations. They are detected by means of scintillation counters, which have an area of up to 100 cm² and which are sometimes used in combination with Geiger-Müller counters. Instruments are being developed that use crystal detectors having areas of 10³-10⁴ cm².

Investigations in gamma-ray astronomy have revealed a uniform (isotropic) cosmic background up to 100 MeV. Radiation has also been observed emanating from the center of our galaxy and from two discrete sources, the Crab Nebula (with a spectrum measured up to 0.5 MeV) and a source in the constellation Scorpio (up to 50 MeV). The source in the Crab Nebula is the remnant of a supernova that exploded in 1054, and the one in Scorpio is the remnant of an outburst of a nova. The nature of the isotropic background as well as of the radiation from the center of our galaxy has not yet been completely explained. Searches are being made for annihilation radiation having an energy of 511 keV, which occurs during the annihilation of an electron-positron pair. The detection of such radiation would seem to indicate the existence of antimatter in the universe. It can be assumed that observations with gamma telescopes of large areas would permit the extension of spectral studies to discrete X-ray sources in the region above 10 keV.

Investigations in gamma-ray astronomy are important for cosmology (observations of hot intergalactic gas), and for determining the nature of the activity in the nuclei of Seyfert galaxies and in quasars, neutron stars, and the discrete sources of galactic and extragalactic X-radiation and gamma radiation. Work in gamma-ray astronomy is being conducted in the USSR, USA, and Japan.

V. G. KURT