Compton Gamma Ray Observatory

Compton Gamma Ray Observatory

Organization	NASA
Wavelength regime	gamma
Orbit height	450 km, 280 mi above ground
Orbit period	90 min, 1.5 h
Launch date	5 April 1991
Deorbit date	4 June 2000
Mass	17 000 kg, 37 500 lb
Other names	none
Webpage	http://cossc.gsfc.nasa.gov/
Physical characteristics
Telescope style	scintillation detectors
Diameter	N/A
Collecting area	varies by instrument
Focal length	N/A
Instruments
BATSE	all-sky monitor
OSSE	pointed detectors
COMPTEL	imaging telescope
EGRET	wide field telescope

The Compton Gamma Ray Observatory (CGRO) 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 Space Technology) in Redondo Beach, CA. 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 (280 miles) in order to avoid the Van Allen radiation belt. It was the heaviest astrophysical payload ever flown at that time at 17000 kg.

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. [1]

Instruments

CGRO carried a complement of four instruments that covered an unprecedented six decades of the electromagnetic spectrum, from 20 kev to 30 GeV. In order of increasing spectral energy coverage:

• Burst and Transient Source Experiment (BATSE) by NASA Marshall Space Flight Center searched the sky for short duration gamma ray bursts (20 to 600 keV) and conducted full sky surveys for long-lived sources. It consisted of 8 detectors, one at each of the satellite's corners (top and bottom). Each detector consisted of both a Large Area Detector and a Spectroscopy Detector.
• Oriented Scintillation Spectrometer Experiment (OSSE) by the Naval Research Laboratory detected gamma rays entering the field of view of any of four detectors, which could be pointed individually, in the 0.05 to 10 MeV range. The four detectors were arranged in pairs of two. During a gamma ray burst event, one detector would take observations of the source, while the other would slew slightly off souce 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.
• Imaging Compton Telescope (COMPTEL) by Max Planck Institute, the University of New Hampshire, Netherlands Institute for Space 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 steradian.
• 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 Center, Max Planck Institute, and Stanford University.

Results

The Compton Gamma Ray Observatory during deployment from STS-37

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 ²⁶Al (a radioactive isotope of Aluminum).
• The OSSE instrument completed the most comprehensive survey of the galactic center, and discovered a possible antimatter "cloud" above the center.
• The BATSE instrument averaged one gamma ray burst event detection per day for a total of approximately 2700 detections.
• The discovery of the first four soft gamma ray repeaters; 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

Main article: GRB 990123

Gamma ray burst 990123 (January 23, 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 4.5 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 pulsar survey and a supernova remnant survey
• The discovery of terrestrial gamma ray sources in 1994 that came from thunderclouds

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 make de-orbiting much more difficult and dangerous. NASA decided with some controversy that a controlled crash was preferable in the interest of public safety, to letting the craft come down on its own. It entered the Earth's atmosphere on 4 June 2000, with debris falling harmlessly into the Pacific Ocean.

See also

• Gamma-ray Large Area Space Telescope
• Great Observatories program
• Swift Gamma-Ray Burst Mission

External links

• NASA Compton Gamma Ray Observatory site
• New Scientist