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Los Alamos National Laboratory Research Quarterly, Fall 2002
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Mysterious Gamma-Ray Bursts

Gamma-ray bursts were discovered in signals from satellites designed and built at Los Alamos. The satellites'   mission was to verify compliance with the nuclear test ban treaty by looking for the x-rays and gamma rays produced by a nuclear explosion in space. Los Alamos scientists published their discovery of gamma-ray bursts in 1973, concluding that the sources of the bursts were cosmic and not terrestrial or solar. The scientists also tried—but failed—to relate the bursts to supernovas.

Burst gamma rays are photons with energies of ten thousand to several million electronvolts. Because of their energies, these gamma rays are highly attenuated by the atmosphere and thus can be detected only in space. (In contrast, the photons of light emitted by some bursts have energies of about 1 electronvolt and pass easily through the atmosphere. These photons can be detected by ground-based instruments such as the RAPTOR observatories.)

Modern satellites detect about one gamma-ray burst a day. A burst can last from a few thousandths of a second to more than 15 minutes. The closest burst, detected on March 29, 2003, was 2 billion light years from Earth, but 10 billion light years is more typical. Such large distances mean the bursts have occurred at the "edge" of the universe, or when it was much younger. For example, the light from a burst 10 billion light years away—which has taken 10 billion years to reach Earth—occurred when the universe was 30 percent of its present age. Gamma-ray bursts thus provide a window on the early universe.

For a time, the bursts' enormous energies puzzled astrophysicists. For example, the gamma-ray burst captured on January 23, 1999, by ROTSE, a robotic predecessor of RAPTOR, was the most luminous celestial object ever observed. (Although the observed brightness of an object decreases as its distance from the observer increases, its intrinsic brightness, or luminosity, is independent of the distance.) Assuming the burst emitted radiation uniformly in all directions, astrophysicists estimated its energy to be about 1032 megatons, or the energy of two solar masses. (Einstein's equation, E = mc2 , equates energy E and mass m, where c is the speed of light.)

However, light measurements of the 1999 burst made more than three days after it occurred supported the idea that the gamma rays and the light were in fact emitted in collinear beams that just happened to shine on Earth, like a searchlight. For a beam scenario, the estimated burst energy was more reasonable—about 1029 megatons, the energy of a typical supernova explosion.

Astrophysicists now believe that a burst's gamma rays are emitted in a tight beam and that its light is emitted more broadly. Thus, if Earth is slightly off the beams' axis, the light flash would be seen on Earth but the gamma-ray burst would not. Such bursts are called orphan gamma-ray bursts. Although orphan bursts have never been observed, astronomers believe they exist, and RAPTOR is expected to find many of them. When it does, astronomers will be able to compare the rates at which orphan bursts and ordinary bursts occur to determine the average width of the gamma-ray beam. This information will shed light on how the bursts are produced.

ROTSE measurements also showed that the light from the 1999 burst faded rapidly in the first 10 minutes, as shown above in the six-frame movie. (These first-ever measurements of a burst's early light were possible only because ROTSE's telescopes were pointed at the event within seconds of the satellite alert.) The magnitude data cast doubt on a generally accepted theory that the shock wave generated by a supernova interacts with the gas surrounding the explosion to produce both light and gamma rays. Instead, the ROTSE data suggested   that the gamma rays are produced closer to the explosion.

Although there had long been hints that gamma-ray bursts and supernovas were connected, there had been no proof of that connection until the "nearby" burst of March 29, 2003. Because the burst was so close and bright (astronomers joked about it casting shadows), scientists were able to measure its light in detail for several weeks. About a week after the burst, the spectral signature of a supernova appeared in the burst's fading afterglow, which proved that gamma-ray bursts and supernovas can be intimately connected. In fact, it is now clear that at least some gamma-ray bursts are produced by supernovas.


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