Introduction to Mysterious Gamma Rays
By early 2000, astronomers had made considerable progress in understanding one of the universe's most puzzling phenomena: gamma-ray bursts. These intense flashes of radiation, which come from all points of the sky, are the most powerful emissions of energy in the universe since the big bang, the explosive event that most astronomers believe gave birth to the universe. For 30 years, researchers using instruments on satellites and powerful ground-based telescopes had scanned the skies for the sources of gamma-ray bursts in a vain attempt to explain them. Finally, in the 1990's, the mysterious bursts began to give up their secrets.
Gamma ray bursts (GRB's) appear without warning about once a day from random directions in the heavens. Gamma rays are the highest-energy form of light in the electromagnetic spectrum, the range of electromagnetic waves that includes visible light as well as ultraviolet rays, X rays, infrared rays, and radio waves. Earth's atmosphere blocks gamma rays from reaching the ground. However, they can be detected by satellites in space that orbit above Earth's atmosphere.
GRB's were discovered in 1967 by United States Vela satellites, which had been launched to monitor global compliance with the nuclear test-ban treaty. By early 2000, astronomers had recorded more than 2,500 such bursts.
Outshining the Rest of the Universe
GRB's are highly variable. Some bursts last less than 1/100 of a second, while others continue for more than 15 minutes. At its peak, a GRB outshines the rest of the universe, as seen in gamma rays from the vicinity of Earth.
For two decades after GRB's were discovered, most astrophysicists believed that they originated within the plane of our Milky Way Galaxy, within 1,000 light-years of Earth. (A light-year is the distance light travels in one year, about 9.5 trillion kilometers [5.9 trillion miles].) That theory, however, was disproved in 1991, when the National Aeronautics and Space Administration (NASA) launched the Compton Gamma Ray Observatory. The observatory's measurements of GRB positions made it clear that the bursts come equally from all directions.
The Compton also measured the bursts' intensities. Some GRB's were bright while others were faint, indicating the possibility of both near and faraway sources for the bursts. After studying these results, some researchers still contended that GRBs have a “local” origin within the Milky Way. Others argued for a “cosmological” theory, according to which bursts arise throughout an enormous region of the universe, extending many millions or billions of light-years from Earth.
Pinpointing the Origin of Gamma-ray Bursts
The question remained unresolved until a joint Italian-Dutch satellite called BeppoSAX was launched in 1996. BeppoSAX carried instruments that could detect both gamma rays and X rays. Each time the gamma-ray monitor detected a burst, researchers used wide-field X-ray cameras on the satellite to search for an X-ray source at the GRB position. (X rays can be imaged better than gamma rays and thus might show the object that produced them.)
After detecting the X-ray afterglow of a gamma-ray burst, BeppoSAX pinpointed its exact location with one of its X-ray telescopes and beamed the information to controllers on the ground. Astrophysicists hoped that eventually they might be able to observe a visible-light afterglow from a burst. If such a glow occurred—and if it lasted long enough for astronomers to record its spectrum—they would be able to measure the distance to the burst.
In 1997, the researchers hit pay dirt. When the BeppoSAX monitor detected a GRB from a position in the direction of the constellation Orion on February 28, one of the wide-field cameras found a bright new source of X rays in the same direction, and an X-ray telescope measured its location. At the University of Amsterdam in the Netherlands, astronomer Jan van Paradijs contacted his graduate student at an optical telescope in the Canary Islands and directed him to observe that position as soon as possible. The student was able to detect an optical afterglow from the GRB, but obtained no further information.
On May 8, a gamma-ray burst spotted by BeppoSAX had both an X-ray afterglow and an optical afterglow that lasted long enough for astronomers at the Keck II Telescope on Mauna Kea, Hawaii, to observe. The astronomers measured the distance to the source of the burst and found that it lay far beyond the Milky Way.
Any remaining doubts about the cosmological origin of GRB's were erased by measurements of a burst that occurred on Dec. 14, 1997. Observers using the Keck telescope calculated that the source of the burst lay more than 10 billion light-years from Earth.
Getting A More Detailed Look At GRB's
On Jan. 23, 1999, an even more sophisticated tool—a ground-based camera called the Robotic Optical Transient Search Experiment (ROTSE—allowed astronomers not only to see a burst's afterglow, but also to watch its rise and fall. Just 22 seconds after the Compton Gamma Ray Observatory registered a burst, ROTSE—located at the Los Alamos National Laboratory in New Mexico—began snapping pictures. It recorded a brief spurt of visible light that reached its peak brightness about 50 seconds after the burst began. This bright light, known as the “prompt optical emission” of a GRB, was millions of times brighter than a typical supernova (exploding star). Astronomers, using the visible afterglow of the 1999 burst, determined that the source of the GRB must be located about 9 billion light-years from Earth.
While tracking the January 23 GRB, astronomers discovered another useful characteristic of the bursts. They detected emissions from the burst in the radio portion of the electromagnetic spectrum. Studies of this radio afterglow showed that it arose from matter hurling outwards from the burst source at close to the speed of light (299,792 kilometers [186,282 miles] per hour), a finding that was consistent with the enormous energy of the bursts.
What Could Produce Such Tremendous Outpourings of Energy?
Although investigators by 2000 had a better idea of where GRB's come from, they continued to puzzle over the cause of these powerful blasts of radiation. Even before GRB's were discovered, physicist Stirling A. Colgate of the Los Alamos National Laboratory predicted that supernovae should emit bursts of gamma rays. Colgate's theory was rejected when the first GRB's were reported, because no supernovae were seen in their general directions. However, a few optical afterglows from later GRB's seemed to be associated with supernovae so distant that they might not have been observed in the 1970's. Nevertheless, Colgate's theory could not explain bursts with the enormous energies of those detected by BeppoSAX.
By early 2000, most researchers believed that GRB's are triggered either when a star with 10 or more times the mass of the sun collapses into a black hole in a hypernova (extremely powerful supernova) or when two neutron stars collide, also forming a black hole. (A neutron star is the small, incredibly dense remnant of a massive star that has perished in a supernova explosion. It is made of neutrons, electrically neutral particles normally found in an atomic nucleus.)
To solve the mystery of gamma-ray bursts, NASA planned to launch a satellite called SWIFT in 2003 that could observe light sources in the gamma-ray, X-ray, ultraviolet, and visible-light portions of the spectrum. Even before then, a small burst-observing satellite called the High Energy Transient Experiment-2 (HETE-2) was to be launched in mid-2000 to continue the work of the Compton Gamma Ray Observatory. The Compton, which had lost one of its steering instruments, was to be destroyed in June as it re-entered the Earth's atmosphere, while technicians could still control the observatory and pick the site of its crash.