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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 Ulysses (1990), 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, the Swift Gamma Ray Burst Explorer, launched in 2004, and the Fermi Gamma-Ray Space Telescope, launched in 2008. Swift detected (2009) an extremely distant gamma-ray burst (more than 13 billion light-years from Earth) that may be associated with the supernova of a blue giant star of the early universe, and Fermi has discovered hundreds of gamma-ray sources.
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. In 2017 the observation of gravitational waves from a neutron-star merger in Hydra and of a coincident GRB confirmed that the collision of two neutron stars creates a GRB and allowed for other observations of the merger optically and by other means.
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 astronomyThe study of radiation from space at the extreme short-wavelength end of the electromagnetic spectrum (less than 0.01 nanometers) and with the largest photon energies (usually exceeding 100 000 electronvolts (eV), i.e. >100 keV). The immense range of gamma-ray energies (see gamma rays) has led to a variety of detection instruments and techniques, including scintillation counters, spark chambers, drift chambers, coded-mask telescopes, Compton telescopes, and gamma-ray solid-state detectors.
Gamma rays from space cannot penetrate the Earth's atmosphere so that γ-ray observations only became possible when instruments could be carried above the atmosphere in satellites. The first cosmic γ-rays were detected with a scintillation counter on the OSO-3 satellite in 1968. More detailed observations confirmed their origin in a narrow band along the galactic plane, being particularly strong within 30° of the galactic center; these observations were provided by the NASA SAS-2 satellite launched in Nov. 1972 and suggested that the γ-rays come from interactions in interstellar gas. The spark-chamber detector on SAS-2, sensitive in the energy band 30–1000 MeV, also found discrete γ-ray sources coincident with two pulsars, the Crab and Vela pulsars.
The European Space Agency's COS-B satellite, launched in Aug. 1975, also carried a large spark chamber and operated successfully for nearly seven years, providing improved exposure and angular resolution of medium-energy (70–5000 MeV) γ-rays. In addition to the Crab and Vela pulsars 22 other galactic sources, plus the bright quasar 3C273, were detected by COS-B. Identification of the remaining γ-ray emitters with known celestial objects has not, however, been possible. Detailed study of the COS-B positional error boxes has shown that none of these medium-energy γ-ray sources can have an X-ray luminosity more than a fraction of the γ-ray luminosity; this indicates that an understanding of their physical nature has to be found in the interpretation of γ-ray data and, furthermore, that they could represent a new class of cosmic object. At least two of the COS-B sources are coincident with molecular cloud regions (Rho Ophiuchi and the Orion molecular cloud). The observed positional correlation between the γ-ray isophotes and the distribution of extinction from these regions suggests γ-ray production by means of particle interactions with the matter of the cloud. The improved sensitivity of instrumentation on the Compton Observatory has shown that a growing number of pulsars are powerful γ-ray emitters (see gamma-ray pulsars; Geminga).
Apart from the localized sources in our Galaxy, a clearly defined band (±15° latitude) of enhanced γ-ray continuum emission lying along the galactic equator has been observed; it has been interpreted as being mainly the result of interaction of cosmic-ray electrons (via bremsstrahlung) and cosmic-ray nuclei (via neutral pion, π0, decay) with the interstellar gas. There is also evidence favouring local structure in the γ-ray emission coincident with the Gould Belt as well as the Doliditz system. The Galaxy is also a rich source of low-energy (0.5–10 MeV) γ-ray emission, which encompasses a diffuse component as well as a number of discrete sources. The galactic center is a powerful emitter of electron-positron annihilation radiation. Detailed studies with germanium detectors of high spectral resolution have revealed that the line is narrow and shows some evidence for three-photon positronium continuum below 511 keV.
The 1.809 MeV line of 26Al has been detected from the general direction of the galactic plane. The emission intensity corresponds to approximately 3 MO of this long-lived (106 year) isotope within our Galaxy. The discovery of the 0.847 MeV γ-ray line from SN 1987A provided the first direct evidence of the explosive synthesis of elements. About 0.075 MO of the radioactive isotope 56Ni were produced in this supernova explosion.
Although the quasar 3C273 was detected as a medium-energy γ-ray source, other types of active galaxies have been discovered to be strong emitters of γ-rays. The γ-ray power output is found to dominate all emissions at other wavelengths. This high luminosity coupled with the timescale of the γ-ray variability (a few months) may be taken as evidence that the emissions are intimately related to the region containing the central power house.
A diffuse, and as far as can be measured isotropic, cosmic γ-ray flux has been observed to extend from 0.1 MeV to more than 1000 MeV. At the present time it is unclear if this cosmic background flux is derived from particle interactions throughout the Universe, or whether it is derived from the contributions of a large number of active galaxies.
A growing number of galactic objects have been detected as very high energy (> 1012 eV) γ-ray emitters. In two cases a neutron star is almost certainly involved and the emission follows the period of pulsations at other wavelengths.
The measurement of γ-ray photons permits the study of the largest transfers of energy occurring in astrophysical processes. The extreme penetrating power of these high-energy photons offers a unique opportunity to probe deeply into the heart of violent galactic and extragalactic systems. The new generation of more sensitive γ-ray telescopes are revealing much new information on both the properties of astronomical objects and the high-energy processes that govern the dynamics of their evolution. See also Compton Gamma Ray Observatory; GRANAT; HETE; INTEGRAL.
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