Gamma-ray Astronomy
Gamma-ray Astronomy
History of Gamma-ray Astronomy
Long before experiments could detect gamma rays emitted by cosmic sources, scientists had known that the universe should be producing these photons. Work by Feenberg and Primakoff in 1948, Hayakawa and Hutchinson in 1952, and Morrison in 1958 had led scientists to believe that a number of different processes occurring in the universe would result in gamma-ray emission. These processes included cosmic ray interactions with interstellar gas, supernova explosions and interactions of energetic electrons with magnetic fields. However, it was not until the 1960s that scientists were able to detect these emissions.Gamma rays coming from space are mostly absorbed by Earth's atmosphere. So gamma-ray astronomy could not develop until it was possible to get the detectors above all or most of the atmosphere, using balloons or spacecraft. The first gamma-ray telescope was carried into orbit on the Explorer-XI satellite in 1961, and picked up fewer than 100 cosmic gamma-ray photons. These appeared to come from all directions in the universe, implying some sort of uniform "gamma-ray background." This background would be expected from the interaction of cosmic rays (very energetic charged particles in space) with gas found between the stars.
Significant gamma-ray emission from our Galaxy was first detected in 1967 by the the gamma-ray detector aboard the OSO-3 satellite. It detected 621 events attributable to cosmic gamma rays. However, the field of gamma-ray astronomy took great leaps forward with the SAS-2 (1972) and the COS-B (1975-1982) satellites. These two satellites provided an exciting view into the high-energy universe, sometimes called the "violent" universe, because the type of events in space that produce gamma rays tend to be explosions and high-speed collisions. The data from the satellites confirmed the earlier findings of the gamma-ray background, produced the first detailed map of the sky at gamma-ray wavelengths, and detected a number of point sources, where the sources of radiation were very concentrated and emanated from a small area. However, the poor resolution of the instruments made it impossible to identify most of these point sources with individual stars or stellar systems.
Perhaps the most spectacular discovery in gamma-ray astronomy came in the late 1960s and early 1970s from a collection of defense satellites that were put into orbit for a reason completely unrelated to astronomy research. Detectors on board the Vela satellite series were designed to detect flashes of gamma rays from nuclear bomb blasts. They began to record bursts of gamma rays, not from the vicinity of Earth, but from deep space. These gamma-ray bursts (GRBs) can last for fractions of a second to minutes, popping off like cosmic flashbulbs from unexpected directions, flickering, and then fading after briefly dominating the gamma-ray sky. Studied for over 25 years with instruments on board a variety of satellites and space probes, including Soviet Venera spacecraft and the Pioneer Venus Orbiter, the sources of these enigmatic high-energy flashes for a while remained a mystery. In one of the most intense debates in modern astrophysics, some scientists claimed that the bursts originate in a halo of neutron stars which surround our Galaxy while others argued that their origins are far beyond the Galaxy, at cosmological distances. This was settled in 1996 when the BeppoSax satellite and the Hubble Space Telescope pinpointed the location of a gamma-ray burst in distant galaxy.
In 1977, NASA announced plans to build a "great observatory" for gamma-ray astronomy. The Compton Gamma-Ray Observatory (CGRO) was designed to take advantage of the major advances in detector technology during the 1980s, and was launched in 1991. The satellite carried four major experiments which greatly improved the spatial and temporal resolution of gamma-ray observations. The CGRO provided large amounts of data which have been used to improve our understanding of the high-energy processes in our Universe. CGRO was de-orbited in June 2000 as a result of the failure of one of its stabilizing gyroscopes.
In November 2004, NASA launched the Swift satellite. Its primary mission is to detect and locate GRBs as quickly as possible, report the position of the burst, then follow up with other observations of that location in the X-ray, UV and visual spectra. On April 13, 2010, NASA's Swift satellite recorded its 500th GRB.
To continue the study of the universe in the gamma-ray spectrum, Swift currently operates in conjunction with the Fermi Gamma-Ray Space Telescope, launched in 2008. Fermi, originally called GLAST (Gamma-ray Large Area Space Telescope), also studies GRBs, as well as blazars, neutron stars, gamma-ray background radiation, supernova remnants, dark matter and more.
Tell me more about gamma-ray detection!
Last Modified: October 2010
