Compton Gamma Ray Observatory
Encyclopedia
The Compton Gamma Ray Observatory (CGRO) was a space observatory
detecting light from 20 KeV to 30 GeV in Earth orbit from 1991 to 2000. It featured four main telescopes in one spacecraft covering x-rays and gamma-rays, including various specialized sub-instruments and detectors. Following 14 years of effort, the observatory was launched on the Space Shuttle Atlantis
, mission STS-37
, on 5 April 1991 and operated until its deorbit on 4 June 2000. It was deployed in low earth orbit
at 450 km (279.6 mi) to avoid the Van Allen radiation belt
. It was the heaviest astrophysical payload ever flown at that time at 17000 kilograms (37,478.6 lb).
The CGRO is part of NASA's Great Observatories series, with the Hubble Space Telescope
, the Chandra X-ray Observatory
, and the Spitzer Space Telescope
. It was the second of the NASA
"Great Observatories
" to be launched to space, following the Hubble Space Telescope. CGRO was named after Dr. Arthur Holly Compton (Washington University in St. Louis
), Nobel prize winner, for work involved with gamma ray
physics. CGRO was built by TRW
(now Northrop Grumman
Aerospace Systems) in Redondo Beach, CA. CRGO was an international collaboration and additional contributions came from the European Space Agency
and various Universities, as well as the U.S. Naval Research Laboratory,
, from 20 keV
to 30 GeV (from 0.02 MeV to 30000 MeV). In order of increasing spectral energy coverage:
(23 January 1999) was one of the brightest bursts recorded at the time, and was the first GRB with an optical afterglow observed during the prompt gamma ray emission (a reverse shock flash). This allowed astronomers to measure a redshift
of 1.6 and a distance of 3.2 Gpc. Combining the measured energy of the burst in gamma-rays and the distance, the total emitted energy assuming an isotropic explosion could be deduced and resulted in the direct conversion of approximately two solar masses into energy. This finally convinced the community that GRB afterglows resulted from highly collimated explosions, which strongly reduced the needed energy budget.
This de-orbit was NASA's first intentional controlled de-orbit of a satellite.
Space observatory
A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects...
detecting light from 20 KeV to 30 GeV in Earth orbit from 1991 to 2000. It featured four main telescopes in one spacecraft covering x-rays and gamma-rays, including various specialized sub-instruments and detectors. Following 14 years of effort, the observatory was launched on the Space Shuttle Atlantis
Space Shuttle Atlantis
The Space Shuttle Atlantis is a retired Space Shuttle orbiter in the Space Shuttle fleet belonging to the National Aeronautics and Space Administration , the spaceflight and space exploration agency of the United States...
, mission STS-37
STS-37
STS-37, the eighth flight of the Space Shuttle Atlantis, was a six-day mission with the primary objective of launching the Compton Gamma Ray Observatory , the second of the Great Observatories program which included the visible-spectrum Hubble Space Telescope, the Chandra X-ray Observatory and the...
, on 5 April 1991 and operated until its deorbit on 4 June 2000. It was deployed in low earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
at 450 km (279.6 mi) to avoid the Van Allen radiation belt
Van Allen radiation belt
The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. It is believed that most of the particles that form the belts come from solar wind, and other particles by cosmic rays. It is named after its discoverer, James...
. It was the heaviest astrophysical payload ever flown at that time at 17000 kilograms (37,478.6 lb).
The CGRO is part of NASA's Great Observatories series, with the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
, and the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
. It was the second of the NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
"Great Observatories
Great Observatories program
NASA's series of Great Observatories satellites are four large, powerful space-based telescopes. Each of the Great Observatories has had a similar size and cost at program outset, and each has made a substantial contribution to astronomy...
" to be launched to space, following the Hubble Space Telescope. CGRO was named after Dr. Arthur Holly Compton (Washington University in St. Louis
Washington University in St. Louis
Washington University in St. Louis is a private research university located in suburban St. Louis, Missouri. Founded in 1853, and named for George Washington, the university has students and faculty from all fifty U.S. states and more than 110 nations...
), Nobel prize winner, for work involved with gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
physics. CGRO was built by TRW
TRW
TRW Inc. was an American corporation involved in a variety of businesses, mainly aerospace, automotive, and credit reporting. It was a pioneer in multiple fields including electronic components, integrated circuits, computers, software and systems engineering. TRW built many spacecraft,...
(now Northrop Grumman
Northrop Grumman
Northrop Grumman Corporation is an American global aerospace and defense technology company formed by the 1994 purchase of Grumman by Northrop. The company was the fourth-largest defense contractor in the world as of 2010, and the largest builder of naval vessels. Northrop Grumman employs over...
Aerospace Systems) in Redondo Beach, CA. CRGO was an international collaboration and additional contributions came from the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
and various Universities, as well as the U.S. Naval Research Laboratory,
Instruments
CGRO carried a complement of four instruments that covered an unprecedented six decades of the electromagnetic spectrumElectromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, from 20 keV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
to 30 GeV (from 0.02 MeV to 30000 MeV). In order of increasing spectral energy coverage:
BATSE
- The Burst and Transient Source Experiment, (BATSE) by NASA's Marshall Space Flight CenterMarshall Space Flight CenterThe George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...
searched the sky for gamma ray burstGamma ray burstGamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...
s (20 to >600 keV) and conducted full sky surveys for long-lived sources. It consisted of eight identical detector modules, one at each of the satellite's corners (left, right; front and back; top and bottom). Each module consisted of both a NaI(Tl) Large Area Detector (LAD) covering the 20 keV to ~2 MeV range, 50.48 cm in dia by 1.27 cm thick, and a 12.7 cm dia by 7.62 cm thick NaI Spectroscopy Detector, which extended the upper energy range to 8 MeV, all surrounded by a plastic scintillator in active anti-coincidence to veto the large background rates due to cosmic rays and trapped radiation. Sudden increases in the LAD rates triggered a high-speed data storage mode, the details of the burst being read out to telemetryTelemetryTelemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
later. Bursts were typically detected at rates of roughly one per day over the 9-year CGRO mission. A strong burst could result in the observation of many thousands of gamma rays within a time interval ranging from ~0.1 s up to about 100 s.
OSSE
- The Oriented Scintillation Spectrometer Experiment, (OSSE), by the Naval Research Laboratory detected gamma rays entering the field of view of any of four detector modules, which could be pointed individually, and were effective in the 0.05 to 10 MeV range. Each detector had a central scintillation spectrometer crystal of NaI(Tl) 12 in (303 mm) in diameter, by 4 in (102 mm) thick, optically coupled at the rear to a 3 in (76.2 mm) thick CsI(Na) crystal of similar diameter, viewed by seven photomultiplier tubes, operated as a phoswichPhoswich DetectorOriginally, "Phoswich detectors were developed to detect low-intensity, low-energy gamma rays, X-rays, as well as alpha and beta particles efficiently in a higher-energy ambient background...
: i.e., particle and gamma-ray events from the rear produced slow-rise time (~1 μs) pulses, which could be electronically distinguished from pure NaI events from the front, which produced faster (~0.25 μs) pulses. Thus the CsI backing crystal acted as an active anticoncidence shield, vetoing events from the rear. A further barrel-shaped CsI shield, also in electronic anticoincidence, surrounded the central detector on the sides and provided coarse collimation, rejecting gamma rays and charged particles from the sides or most of the forward field-of-view (FOV). A finder level of angular collimation was provided by a tungston slat collimator grid within the outer CsI barrel, which collimated the response to a 3.8° x 11.4° FWHM rectangular FOV. A plastic scintillator across the front of each module vetoed charged particles entering from the front. The four detectors were typically operated in pairs of two. During a gamma-ray source observation, one detector would take observations of the source, while the other would slew slightly off source to measure the background levels. The two detectors would routinely switch roles, allowing for more accurate measurements of both the source and background. The instruments could slew with a speed of approximately 2 degrees per second.
COMPTEL
- The Imaging Compton Telescope, (COMPTEL) by the Max Planck Institute for Extraterrestrial PhysicsMax Planck Institute for Extraterrestrial PhysicsThe Max Planck Institute for Extraterrestrial Physics is a Max Planck Institute, located in Garching, near Munich, Germany.In 1991 the Max Planck Institute for Physics and Astrophysics split up into the Max Planck Institute for Extraterrestrial Physics, the Max Planck Institute for Physics and the...
, the University of New HampshireUniversity of New HampshireThe University of New Hampshire is a public university in the University System of New Hampshire , United States. The main campus is in Durham, New Hampshire. An additional campus is located in Manchester. With over 15,000 students, UNH is the largest university in New Hampshire. The university is...
, Netherlands Institute for Space ResearchNetherlands Institute for Space ResearchSRON Netherlands Institute for Space Research is the Dutch expertise institute for space research. The Institute develops and uses innovative technology for groundbreaking research in space, focusing on astrophysical research, Earth science and planetary research...
, and ESA's Astrophysics Division was tuned to the 0.75-30 MeV energy range and determined the angle of arrival of photons to within a degree and the energy to within five percent at higher energies. The instrument had a field of view of one steradianSteradianThe steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane...
. For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would Compton scatterCompton scatteringIn physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
in a forward detector module, where the interaction energy E1, given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of a second layer of scintillators to the rear, where its total energy, E2, would be measured. From these two energies, E1 and E2, the Compton scattering angle, angle θ, can be determined, along with the total energy, E1 + E2, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The vector, V, connecting the two interaction points determined a direction to the sky, and the angle θ about this direction, defined a cone about V on which the source of the photon must lie, and a corresponding "event circle" on the sky. Because of the requirement for a near coincidence between the two interactions, with the correct delay of a few nanoseconds, most modes of background production were strongly suppressed. From the collection of many event energies and event circles, a map of the positions of sources, along with their photon fluxes and spectra, could be determined.
| Comparison | |||||||
| Instrument | Observing | ||||||
|---|---|---|---|---|---|---|---|
| BATSE | 0.02 - 8 MeV | ||||||
| OSSE | 0.05 - 10 MeV | ||||||
| COMPTEL | 0.75 - 30 MeV | ||||||
| EGRET | 20 - 30 000 MeV | ||||||
EGRET
- The Energetic Gamma Ray Experiment Telescope, (EGRET) measured high energy (20 MeV to 30 GeV) gamma ray source positions to a fraction of a degree and photon energy to within 15 percent. EGRET was developed by NASA Goddard Space Flight CenterGoddard Space Flight CenterThe Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
, the Max Planck Institute for Extraterrestrial PhysicsMax Planck Institute for Extraterrestrial PhysicsThe Max Planck Institute for Extraterrestrial Physics is a Max Planck Institute, located in Garching, near Munich, Germany.In 1991 the Max Planck Institute for Physics and Astrophysics split up into the Max Planck Institute for Extraterrestrial Physics, the Max Planck Institute for Physics and the...
, and Stanford UniversityStanford UniversityThe Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
. Its detector operated on the principle of electron-positronPositronThe positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
pair productionPair productionPair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
from high energy photons interacting in the detector. The tracks of the high-energy electron and positron created were measured within the detector volume,and the axis of the V of the two emerging particles projected to the sky. Finally, their total energy was measured in a large calorimeterCalorimeter (particle physics)In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles' energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its...
scintillation detector at the rear of the instrument.
Results
Basic results
- The EGRET instrument conducted the first all sky survey above 100 MeV. Using four years of data it discovered 271 sources, 170 of which were unidentified.
- The COMPTEL instrument completed an all sky map of (a radioactive isotope of aluminumIsotopes of aluminiumAluminium has 22 known isotopes from 21Al to 42Al and 4 known isomers. Only 27Al and 26Al occur naturally, however 27Al has a natural abundance of 99.9+ %. Other than 26Al, all radioisotopes have half-lives under 7 minutes, most under a second. Standard atomic mass is 26.9815386 u...
). - The OSSE instrument completed the most comprehensive survey of the galactic center, and discovered a possible antimatterAntimatterIn particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
"cloud" above the center. - The BATSE instrument averaged one gamma ray burst event detection per day for a total of approximately 2700 detections. It definitively showed that the majority of gamma-ray bursts must originate in distant galaxies, not nearby in our own Milky WayMilky WayThe Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, and therefore must be enormously energetic. - The discovery of the first four soft gamma ray repeatersSoft gamma repeaterA soft gamma repeater is an astronomical object which emits large bursts of gamma-rays and X-rays at irregular intervals. It is conjectured that they are a type of magnetar or, alternatively, neutron stars with fossil disks around them....
; these sources were relatively weak, mostly below 100 keV and had unpredictable periods of activity and inactivity - The separation of GRBs into two time profiles: short duration GRBs that last less than 2 seconds, and long duration GRBs that last longer than this.
GRB 990123
Gamma ray burst 990123GRB 990123
GRB 990123 is a gamma-ray burst that occurred on January 23, 1999. It was the first GRB for which a simultaneous optical flash was detected. GRB 990123 is one of the most distant distinct landmarks in our map of the universe....
(23 January 1999) was one of the brightest bursts recorded at the time, and was the first GRB with an optical afterglow observed during the prompt gamma ray emission (a reverse shock flash). This allowed astronomers to measure a redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
of 1.6 and a distance of 3.2 Gpc. Combining the measured energy of the burst in gamma-rays and the distance, the total emitted energy assuming an isotropic explosion could be deduced and resulted in the direct conversion of approximately two solar masses into energy. This finally convinced the community that GRB afterglows resulted from highly collimated explosions, which strongly reduced the needed energy budget.
Miscellaneous results
- The completion of both a pulsarPulsarA pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
survey and a supernova remnantSupernova remnantA supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...
survey - The discovery of terrestrial gamma ray sources in 1994 that came from thundercloudsCumulonimbus cloudCumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...
De-orbit
After one of its gyroscopes failed, the observatory was deliberately de-orbited. At the time, the observatory was still operational, however the failure of another gyroscope would have made de-orbiting much more difficult and dangerous. With some controversy, NASA decided in the interest of public safety that a controlled crash was preferable to letting the craft come down on its own at random. Unlike the Hubble Space Telescope, it was not designed for on-orbit repair and refurbishment. It entered the Earth's atmosphere on 4 June 2000, with the debris that did not burn up falling harmlessly into the Pacific Ocean.This de-orbit was NASA's first intentional controlled de-orbit of a satellite.