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Gamma Ray Burst

Gamma ray bursts are brief, seconds-long, blasts of radiation of mysterious origin that, in nature, seem to come from the depths of interstellar space. Bursts of gamma radiation also have been measured coming from severe thunderstorms and are a component of nuclear bomb detonation.

The accidental discovery of cosmic gamma ray bursts was confirmed in 1973. Ten years earlier, the United States Air Force had launched the first in a series of satellites that were intended to monitor the effectiveness of the Nuclear Test Ban Treaty. By signing this treaty, nations of the world agreed not to test nuclear devices in the atmosphere of Earth or in space. The Vela satellites (from the Spanish verb velar, which means "to watch") were part of a research and development program that had the goal of developing the technology to monitor nuclear tests from space. Along with a variety of optical and other instruments, the satellites carried x ray, gamma ray, and neutron detectors. The x-ray detectors were intended to sense the flash of a nuclear blast. Although most of the energy of a bomb detonated in space would be directly visible as an x-ray flash, a gamma ray burst at the same time would provide confirmation of a nuclear event. A further confirmation would come from the detection of neutrons. The Vela designers knew that detonating a nuclear bomb behind a thick shield or on the far side of the moon would effectively hide the initial flash of x rays from the satellites' view. But the gamma ray detectors provided a way around this because they could measure the radiation from the cloud of radioactive material blown out from a nuclear blast. This could not be completely shielded from view as it rapidly expanded outward. The Vela satellites could easily detect these gamma rays even if the detonation took place behind the moon, out of direct view of the satellites' x-ray detectors.

By 1969, a number of gamma ray bursts had been detected that were clearly not caused by nuclear explosions on or near the earth. It was concluded that these were "of cosmic origin," and more accurate observations showing the actual directions of future bursts were collected. Finally, the discovery of these cosmic gamma ray bursts was announced in 1973. It has been found that if we could see gamma rays, every day powerful explosions would illuminate the sky. They come from random directions and their cause is not completely understood. Until recently it was even uncertain whether they come from our own solar system or from as far away as the edge of the known universe.

In 1991 the National Aeronautics and Space Administration (NASA) launched the Compton Gamma Ray Observatory, carrying an instrument called the Burst and Transient Source Experiment (BATSE). Specifically designed to study gamma ray bursts, BATSE has added much new information about their origin and distribution in the universe. By studying the information BATSE can gather, scientists hope to determine what causes them. A gamma ray burst detected on January 23, 1999—GRB 990123—was the first to be observed as a visual object as well as in the gamma ray region. This has given scientists an even better look at these mysterious objects, giving clues to the structure of the explosion. Even though the object was nine billion light years away, the light was so bright observers on Earth could see it with a pair of binoculars.

Today, a cosmic gamma ray burst seems like a strange interstellar traffic accident, with material flowing out from the explosion at different speeds. Because of this, some material from the explosion collides with other parts of the expanding shell. This causes "pile-ups" of material that create shock waves. These, in turn, generate energy at various wavelengths. Scientists are currently trying to untangle these clues to better understand the mechanics of the explosion, but it is now clear that in addition to gamma rays, GRBs also emit light, large amounts of x rays, and other radiation.

Clint Hatchett

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