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Unraveling the Mysteries of Gamma Ray Bursts

VLA Image of Gamma Ray Burster

Gamma Ray Bursts (GRBs) were discovered in 1967 by satellites designed to monitor compliance with the atmospheric nuclear test ban treaty. These short-lived outbursts of the most energetic electromagnetic radiation remained one of the biggest mysteries in astrophysics for nearly three decades thereafter. For most of that period, the bursts' positions in the sky were known only with limited precision, making study of them by ground-based optical and radio telescopes impossible.

With the limited information available, scientists could not determine with confidence even such basic information as the distances from Earth of GRBs. Without such fundamaental details, scientists were not able to learn the nature or cause of the bursts. Because of the distance uncertainty, for example, astronomers did not know if GRBs were in our own Solar System, our Galaxy, or in the distant Universe. In this vacuum of knowledge, speculation led to numerous, widely-divergent theories that attempted to explain the bursts.

In 1997, the Italian-Dutch satellite BeppoSAX provided ground-based observers with rapid news and more-precise positional information about GRBs. On May 8, 1997, BeppoSAX detected a Gamma Ray Burst, and VLA observers Dale Frail of NRAO and Shri Kulkarni of Caltech discovered radio emission coming from this object on May 13. "The mere discovery of radio emission from this gamma ray burst rules out some theoretical models," Frail said at the time.

Subsequently, the VLA has detected the radio "afterglow" of several other gamma ray bursts. While spectroscopy done at optical observatories has shown that GRBs are at great distances, beyond our own Milky Way Galaxy, VLA observations have revealed the size of the fireball and the speed of its expansion. The May 8, 1997, GRB, for example, was only a tenth of a light-year across when first detected and expanded at very nearly the speed of light.

The VLA's ability to locate GRBs in the sky with pinpoint precision has helped astronomers at other observatories to locate GRB afterglows that they otherwise might have missed. The image here shows the GRB of March 29, 1998. The position determined by the VLA was provided to optical and infrared observers, who had failed to find the object, but then, armed with the precise information on its location, found it on images they already had made.

After three decades of mystery, astronomers now know that GRBs, the most violent events in the current Universe, occur in galaxies far from Earth. In addition, the VLA has provided strong evidence that these tremendous explosions occur in dusty areas of those galaxies, where it is likely that young stars are located within the clouds of dust and gas from which they formed. This evidence supports the theory that GRBs result from a "hypernova," the explosive death of a very massive star that collapses and forms a black hole.

Radio telescopes are the only instruments presently capable of measuring the size of a GRB fireball. In addition, while GRB "afterglows" fade quickly at other wavelengths, the VLA has been able to follow an afterglow for more than a year, tracking changes in its intensity and other characteristics. "These observations indicate the extraordinary importance of radio astronomy for providing information that can be gained in no other way about one of the frontier areas of astrophysics," said Hugh Van Horn, Director of the National Science Foundation's Division of Astronomical Sciences.

The study of GRB afterglows has opened up "a new era in the studies of gamma ray bursts," wrote Princeton University theorist Bohdan Pacynski. The VLA, along with instruments working at other wavelengths, has made it possible to study the physics of these objects for the first time.

GRB Visibility at Different

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