A Brief History of Gamma-ray Astronomy

Long before they could detect Gamma-rays, scientists suspected this type of radiation was produced throughout the universe. To identify these emissions, Earth-bound detectors needed to be placed outside our atmosphere, which typically absorbs gamma rays. In 1961, the Explorer XI satellite carried the first gamma-ray telescope into Earth's orbit. This instrument gathered fewer than 100 cosmic Gamma-ray photons, Gamma-ray astronomy was born. The information provided is based on an article by Jeremy McGovern.

Explorer XI

Launch Year: 1961
Agency: NASA

Explorer XI
NASA

Carrying a telescope built by Massachusetts Institute of Technology (MIT) scientists William L. Kraushaar and George W. Clark, Explorer XI was the first satellite dedicated to gamma-ray astronomy. Placed in an eccentric orbit around Earth, the satellite searched for the signs of cosmic-ray interaction with interstellar material. Because Explorer could not be actively directed, the spacecraft rotated end over end, providing a rough scan of the celestial sphere. The satellite detected 22 cosmic gamma-ray events before its power source deteriorated.

For more information: http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/explorer11.html

The Vela Program

Vela 5b
NASA

The Vela program was not designed to carry out scientific investigations. Rather, it was initiated to verify the Limited Test Ban Treaty. That treaty had been negotiated between the United States and the Soviet Union and signed in 1963. Its prime purpose was to put a halt to testing of nuclear weapons in space and in Earth\'s atmosphere. Vela was a set of satellites, launched in pairs in the years 1963 to 1970, that orbited far above the Earth and kept watch for clandestine tests. The program was run by the Department of Defense. What makes Vela interesting from a astrophysical perspective is that the satellites were the first to detect gamma ray bursts. The first of these detections occurred in 1967, but because of the secret nature of the satellite program the detections remained classified until 1973. So even though the Vela satellites were not built to do gamma ray astronomy, they still play a role in the history of the field.

Cos-B

Cos-B
ESA

Launched from California, Cos-B was ESA's first satellite dedicated to a single experiment, in this case, detecting gamma rays. The Caravane Collaboration, a collection of research laboratories throughout Western Europe, directed the project. Over six years, Cos-B provided important gamma-ray data, most significantly the first complete map of the Milky Way in gamma rays.

Granat
Russian Space Agency

Granat was used to study both interstellar x rays and gamma rays. The instruments on board were developed by Russia and France. For the first few years of its nine years of service, the satellite made targeted observations. In 1994, Granat began operating in survey mode because its fuel supply was exhausted so the spacecraft could no longer aim. Besides its cosmic ray detections, a highlight of the mission is its deep imaging of the galactic center.

Compton Gamma-ray Observatory

CGRO
NASA

Compared to Explorer XI's weight of roughly 82 pounds, CGRO was a behemoth at 17 tons, making it the largest astrophysical payload of its time. Gamma-ray instruments comprised almost half of the satellite's bulk. By using these larger telescopes, CGRO could gather more gamma-ray photons than previous satellites. The instruments studied photon energies ranging from 20,000 electron volts (20 keV) to more than 30 billion electron volts (30 GeV). CGRO's productive career lasted until 2000, when NASA directed the satellite into Earth's atmosphere. Before its death, CGRO showed us that our universe is a violent and rapidly changing place with its detection of gamma rays, identification of gamma-ray bursts, and the discovery of a new class of quasars. A number of European countries in association with the ESA contributed to this mission\'s success.

Initially, Gamma-rays appeared to come primarily from the Milky Way galaxy. Generalized Gamma-ray emission from the galaxy would be produced by cosmic ray interactions with the dust and gas in the galactic plane. Some pulsars and supernova remnants were also identified as Gamma-ray sources. The Compton Gamma-ray Observatory (CGRO) discovered point sources of Gamma-rays outside the plane of the galaxy. These sources were identified with a special type of active galaxy now known as blazars. CGRO ultimately detected several dozen blazars associated with faint optical sources. It is also now realized that there is a uniform background Gamma-ray emission from the entire sky. At the present time (2004) the source of this emission is unknown.

Yohkoh

Yohkoh
ISAS

Japanese for "sunbeam," Yohkoh studied x rays and gamma rays produced by the sun. A gamma-ray detector from NASA monitored gamma-ray emissions. The satellite functioned reliably until December 14, 2001. During the annular eclipse that day, Yohkoh fell into the shadow of the eclipsed sun, causing the spacecraft to discharge its batteries and lose its ability to point toward the sun.

Fermi Gamma-ray Space Telescope
(GLAST)

Fermi GST
NASA

The Fermi Gamma-ray Space Telescope (originally GLAST) was launched on June 11, 2008. Following a successful commissioning period of approximately 60 days' duration, it has begun a one-year all sky survey; the initial data obtained in the four-day first light images had already equaled the sensitivity of one year of EGRET observations, and with better angular resolution. Fermi is also capable of obtaining pointed observations. It is expected that the Fermi Gamma-ray Space Telescope will detect and document several thousand new blazars. FGST also has burst detectors with sensitivity comparable to those of Swift, and these are now detecting approximately one GRB per day.

Swift

Swift
NASA

Swift was launched in November, 2004 to investigate gamma-ray bursts. When it detects a burst, Swift is able to determine its precise position, at the arcminute level, in a matter of seconds. It then slews over and begins observing with its onboard UV-optical and x-ray telescopes, usually in about 90 seconds or less (It's no accident that the satellite is named Swift). These instruments can determine burst positions at the arcsecond level. They can also measure burst and afterglow light curves. All this information is transmitted to the ground in real time and relayed to ground-based observers who are then able to begin observing the burst as quickly as possible. Typically, Swift detects three or four bursts a week, though the rate varies. To date (January, 2008) Swift has detected nearly 300 gamma ray bursts. Because Swift has greatly increased the numbers of GRB for which optical afterglows are observed, and even for which spectra are obtained, it has had a major impact on our understanding of these powerful and enigmatic events.

INTEGRAL

Integral
ESA

Integral's telescope possesses two imagers that can scan 20,000 to 10 million electrons, making it the most sensitive gamma-ray observatory ever at the time of its launch. Astronomers expect Integral will be key in not only gamma-ray detections, but also in providing clues about the explosions from which these high-energy photons originated.

HETE-2

HETE-2
NASA and MIT

High Energy Transient Explorer 2 was designed to detect and localize transient X-ray and Gamma-ray phenomena such as Gamma-ray Bursts (GRBs). Three years after the malfunction of the HETE launch vehicle, astronomers in France, Italy, Japan, and the United States collaborated for a second attempt. HETE is designed to detect and localize gamma-ray bursts. HETE was successfully launched and began routine detection of bursts in 2002. In October 2002 HETE detected a gamma-ray burst that lasted for 100 seconds. This discovery led to exceedingly detailed observations of a gamma-ray burst. HETE has now detected several bursts for which optical afterglows have been detected and followed.