June 17, 1997
Gamma Ray Bursts Decoded?
Recent Observations Help Unravel Decade-Old Mystery
After foiling astronomers for years, the enigmatic gamma ray bursts are finally starting to shed their veil of mystery, thanks to a prodigious world-wide observational effort. In my January 3 article, I elaborated on our dreadful lack of understanding of gamma ray bursts, and I discussed hypotheses for their nature ranging from comet collisions within the solar system, to neutron star-neutron star collisions in distant galaxies. Within the last months, the Dutch and Italian satellite Beppo-SAX, along with a multi-national collaboration of optical and radio telescopes, has all but confirmed that the bursts are at cosmological (not solar system) distances, a view most notably advocated over the years by Bohdan Paczynski of Princeton.
Beppo-SAX is a simple but clever instrument specifically designed to locate gamma ray bursts to a very precise angular position on the sky (with errorboxes of order arcminutes). First, a 40 degree wide gamma-ray detector waits for a burst within its field of view. When a burst goes off, high energy X-ray detectors look at the same part of the sky the burst came from. The X-ray detectors are able to pinpoint the burst in the sky, a feat impossible with gamma rays which are simply too energetic to be reflected into a focus by any conceivable material. Once Beppo-SAX determines the position, the Dutch and Italian team issues a world-wide telegram to the astronomical community, so that all possible telescopes can monitor the event in optical and radio light.
Last month, just such an event occurred, and detailed optical observations with many telescopes followed the burst over several days. The Keck telescope group reported that the optical counterpart to the gamma-ray burst had absorption features in its spectrum at a redshift of 0.8, a distance of order 10 billion light-years. Now the absorption could come from a cloud of gas closer to us than the gamma ray source, or within the source itself, but either way, the source has to occur at a redshift of at least 0.8, laying to rest a 15-year old (hotly contested) controversy over the distance of the gamma-ray bursts.
Now that we know the distance, we have to figure out what in the world makes so much energy so fast that we can see it from the other side of the universe. The prevailing theory is some sort of expanding fireball, presumably resulting from the collision of massive objects, such as neutron stars; however other progenitors have been suggested, and by no means ruled out. The most rudimentary predictions of the basic fireball model, advanced by Eli Waxman (Institute for Advanced Study) and Martin Rees (Cambridge), have been confirmed by the rough temporal behavior of the light emitted in the X-ray, optical and radio; however the detailed time behavior and spectra of these objects remains unexplained. With roughly one new burst every month, astronomers hope to develop a statistical of burst properties within the coming years, which promises, finally, to shed some understanding on these energetic enigmatic fireballs.
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