About the Compton Gamma Ray Observatory
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The Compton Gamma Ray Observatory (GRO) is a sophisticated satellite observatory dedicated to observing the high-energy Universe. It is the second in NASA's program of orbiting "Great Observatories", following the Hubble Space Telescope. While Hubble's instruments operate at visible and ultraviolet wavelengths, Compton carries a collection of four instruments which together can detect an unprecedented broad range of high-energy radiation called gamma rays. These instruments are the Burst And Transient Source Experiment (BATSE), the Oriented Scintillation Spectrometer Experiment (OSSE), the Imaging Compton Telescope (COMPTEL), and the Energetic Gamma Ray Experiment Telescope (EGRET).
These four instruments are much larger and more sensitive than any gamma-ray telescopes previously flown in space. The large size is necessary because the number of gamma-ray interactions that can be recorded is directly related to the mass of the detector. Since the number of gamma-ray photons from celestial sources is very small compared to the number of optical photons, large instruments are needed to detect a significant number of gamma rays in a reasonable amount of time. The combination of these instruments can detect photon energies from 20 thousand electron volts (20 keV) to more than 30 billion electron volts (30 GeV).
The table of instrument capabilities for the four experiments describes their fields of view, sensitivities to continuum and line emissions, and angular and energy resolutions. An appreciation of the purpose and design of Compton's four instruments is gained from understanding that above the energies of X-ray photons (~10 keV - about 10,000 times the energy of optical photons) materials cannot easily refract or reflect the incoming radiation to form a picture. Hence, alternative methods are required to collect gamma-ray photons and thereby image sources in the sky. At gamma-ray energies, three methods are currently used, sometimes in combination: (1) partial or total absorption of the gamma ray's energy within a high-density medium, such as a large crystal of sodium iodide, (2) collimation using heavy absorbing material, to block out most of the sky and realize a small field of view, and (3) at sufficiently high energies, utilization of the conversion process from gamma rays to electron-positron pairs in a spark chamber, which leaves a telltale directional signature of the incoming photon.
The Compton Observatory has a diverse scientific agenda, which includes studies of very energetic celestial phenomena: solar flares, gamma-ray bursts, pulsars, nova and supernova explosions, accreting black holes of stellar mass, quasar emission, and interactions of cosmic rays with the interstellar medium.
Many exciting discoveries have been made by the instruments on Compton, some previously expected and some completely surprising. The all-sky map produced by EGRET is dominated by emission from interactions between cosmic rays and the interstellar gas along the plane of our Galaxy, the Milky Way. Some point sources in this map are pulsars along the plane. Seven pulsars are now known to emit in the gamma-ray portion of the spectrum, and five of these gamma-ray pulsars have been discovered since Compton was launched. The Crab and Geminga pulsars are found near the Galactic anticenter, on the extreme right side of the EGRET all-sky map. Keep in mind that, since gamma-ray instruments have angular resolutions of the order of 1 degree or larger, point sources in this map will look slightly extended. One of the major discoveries made by EGRET is the class of objects known as blazars - these are quasars that emit the majority of their electromagnetic energy in the 30 MeV to 30 GeV portion of the spectrum. These objects, which are at cosmological distances, are sometimes seen to vary on timescales of days.
An all-sky map made by COMPTEL illustrates the power of imaging in a narrow band of gamma-ray energy, in the light of radioactive aluminum 26. This map reveals unexpectedly high concentrations of this particular isotope in small regions. In a COMPTEL image of the Galactic anticenter, several interesting objects are visible, including two pulsars, a flaring black hole candidate and a gamma-ray blazar.
In another map of the Galactic center region, scanning observations made by OSSE reveal gamma-ray radiation from the annihilation of positrons and electrons in the interstellar medium, another line emission. The spectrum of a solar flare recorded by OSSE yields direct evidence accelerated particles smashing into material on the sun's surface, exciting nuclei which then radiate in gamma rays.
One of BATSE's primary objectives is the study of the mysterious phenomenon of gamma-ray bursts - brief flashes of gamma rays which occur at unpredictable locations in the sky. BATSE's all sky map of burst positions shows that, unlike Galactic objects which cluster near the plane or center of the Galaxy, these bursts come from all directions. A cosmological origin (i.e., well beyond our Galaxy) has now been established. Burst light curves suggest that a chaotic phenomenon is at work; no two have ever appeared exactly the same. An average light curve for bright and dim bursts is consistent with the explanation that bursts are at cosmological distances: the dim ones, which presumably are farther away, are stretched more in cosmic time than are the bright ones, as the events participate in the general expansion of the Universe. BATSE also has the capability to image the sky, using the Earth as an occulting disk, via a technique called Radon transforms.
The Laboratory for High Energy Astrophysics at Goddard Space Flight Center has three areas of involvement with Compton: the team in charge of the EGRET instrument, part of the spectroscopy analysis team for the BATSE instrument, and the Compton Observatory Science Support Center, which supports the Guest Investigator program and archival analysis research.
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