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Gamma rays, which are at the highest end of the electromagnetic spectrum, are intriguing because scientists believe they may be a reliable record of cosmic change and evolution. The exploration of these gamma rays offers answers to perplexing questions about the explosive and dynamic physical process that make the Universe work the way it does. Exploration also answers such important questions as to the structure and dynamics of the Milky Way and other galaxies, the nature of pulsars, quasars, black holes and neutron stars, and the origin and history of the Universe itself.
On April 5, 1991, the Space Shuttle Atlantis (OV-104) placed the Arthur Holly Compton Gamma-Ray Observatory into a low-Earth circular orbit (altitude 270 miles, or 450 kilometers) at an inclination of 28.5 degrees from the equator. The purpose of the mission is to obtain gamma-ray measurements over the entire celestial sphere, with significantly improved angular resolution and sensitivity over previous gamma-ray space missions. The Compton Observatory has four highly sophisticated instruments capable of making simultaneous observations spanning more than five decades of energy. Data and information obtained from the observatory is providing great new insights into the young and growing science of gamma-ray astronomy.
Previous gamma-ray missions consisted of small spacecraft equipped with specialized instruments, such as NASA's second Small Astronomy Satellite (SAS-2), the European Celestial Observation Satellite (COS-B), NASA's High Energy Astronomy Observatories (HEAOs 1 and 3) and the Solar Maximum Mission. Information from these missions confirmed the potential for further exploration of the gamma-ray Universe, using large sophisticated instruments covering a broad range in the high-energy spectrum.
Gamma rays are the highest energy radiations in the electromagnetic spectrum, ranging from tens of thousands to tens of billions of electron volts (eV). An eV is a measure of the amount of energy imparted to an electron when subjected to an electrical potential of one volt. By contrast, visible light corresponds to only a few electron volts. These gamma rays provide a way to study some of the primary forces of change in the astrophysical processes. Gamma rays are thought to have originated with the "Big Bang" and subsequent expansion of the Universe, and the many energetic phenomena we witness today. Through these gamma-ray observations we may witness the birth of elements and deaths of stars, gain clues into the mysteries of quasars, pulsars, neutron stars and peer into the space/time precipice of a black hole. In all these cases, large amounts of energy are released, and gamma rays are produced. A classical example of this process is the excitation of the nucleus of an atom (which consists of protons and neutrons), causing it to move into a higher energy state. As the nucleus drops back to its original state, one or more gamma rays are emitted. These gamma rays will have the precise amount of energy as the difference between the nucleus' low-energy state and its high-energy, or excited, state. Gamma rays, the radiation that reveals the most energetic phenomena in the Universe, are particularly interesting because they are our only source of information for some events. These gamma rays from cosmic sources are able to travel across galaxies and most of the Universe without being absorbed, but cannot penetrate the Earth's atmosphere except at extremely high energies. The information they hold is not available to observatories on the ground and must be observed from space.
Weighing more than 17 tons, the Compton Observatory was the heaviest civilian spacecraft ever deployed from the shuttle. The spacecraft was built by TRW in Redondo Beach, Calif., and measures 70 feet (21 meters) between the tips of its solar arrays. These solar arrays provide the 1,800 watts of electrical power required for operation of the observatory. The spacecraft has already exceeded its minimum life of two years, but is expected to operate for at least six years.
During the first 15 months of operation, a detailed survey of the gamma-ray Universe has been obtained. The instruments also are viewing, in much greater depth, intriguing objects identified by the survey. This much longer observing time dedicated to a particular source or region is providing a much more detailed characterization of its properties and the processes responsible for its gamma-ray production.
Data is collected 24 hours a day. Because the signals from gamma-ray emitting objects are very weak, the observatory is pointing at the science targets and collecting data for periods of up to 14 days.
Attitude adjust maneuvers are performed to focus the instruments on the next target, and data collection begins again. The observatory contains four science instruments: BATSE, OSSE, COMPTEL, and EGRET, which make measurements spanning the broadest range of energies ever achieved from space.
The Naval Research Laboratory in Washington, D.C., designed OSSE to detect nuclear-line radiation and emissions associated with low-energy gamma-ray sources anywhere in the sky. This information is contributing to the understanding of many types of science targets such as novae, supernovae, neutron stars, black holes, pulsars, quasars, and X-ray sources.
This telescope, developed as a joint venture of Germany, the Netherlands, the European Space Agency, and the United States, has the imaging capability, broad effective aperture, and low-background that are needed to study gamma-ray characteristics of point sources, diffuse emission from the galaxy, cosmic diffuse flux, and broadened line emissions. It determines the direction of arrival, as well as the energy of, gamma-ray photons from one to 30-million electron volts.
This instrument is the result of collaboration by scientists at GSFC, Stanford University, Grumman Aerospace and Germany. EGRET is designed to measure the higher energy gamma rays, up to 30 billion electron volts. It can measure the direction of a point source to a fraction of a degree and determine the spectrum of gamma-ray emissions. Emissions from other galaxies are being examined to study their structure and dynamics and the distribution of cosmic rays within them.
In 1979, a gamma-ray burst was observed near the Large Magellanic Cloud, our nearest galactic neighbor. In just 1/10 second, it unleashed more gamma-ray energy than our sun can pump out in every form over the next 1,000 years. Compton is looking at such bursts in an attempt to understand the origin of these mysterious phenomena, and is examining other kinds of transient emissions.
Explosions of stars are known to be sites of element formation, or nucleosynthesis. Compton is observing gamma-ray emission lines for details of this process in supernovae, and is gathering data to test current theory that cosmic rays are created in supernovae and are accelerated by shock waves.
Compton is providing new insights into the evolution of galaxies over their entire dynamic range, from the first fluctuations in the Universe billions of years ago through the more recent emergence of active galactic nuclei. Compton is aiding in the study of galactic structure, dynamic balance between the binding forces of cosmic ray gas, kinetic motion of interstellar medium, and galactic magnetic fields.
Pulsars are thought to be remnants of supernova explosions. Since the first pulsar was discovered in 1968, we now have located more than 500 of these rapidly spinning neutron stars. A pulsar appears to be able to produce more energy over its lifetime than the explosion which created it, and Compton is shedding new light on the perplexities of this phenomena.
In addition, Compton is focusing on gamma-ray luminosities thousands of times greater than those in the Milky Way. It is exploring unusually active systems such as intense radio galaxies and Seyfert galaxies or BL Lacertae objects and quasars, all of which emit far more energy than a normal galaxy and, for an unknown reason, vary in brightness. Quasars are curious objects characterized by a very high output of energy mostly coming from the centers of the galaxies. The instruments on Compton, with a sensitivity 10 times greater than earlier instruments, are scanning the heavens for new information on active galaxies.
The observatory can monitor the very high temperature emissions from the vicinity of stellar black holes. Compton's search for small primordial black holes are helping resolve questions relating to the large-scale distribution of matter, which is one of the keys to testing theories of an expanding Universe beginning with a colossal explosion, or "Big Bang."
Examples of discoveries by Compton include: