Last year, the study of gamma-ray bursts (GRBs) reached a crossroads. As 1997 began, the Compton Gamma Ray Observatory (GRO) had logged more than 1,700 GRBs, and most astronomers believed te bursts took place billions of light-years away. Yet no one had convincingly tied even one GRB to a recognisable astronomical object.
In part, this was because GRO could say only roughly which part of the sky each burst came from — its GRB positions are good only to a few degrees, and hordes of possible counterparts lie within such large sky patches. A few bursts had been detected by more than one spacecraft, and this enabled them to be pinpointed by triangulation. But no unambiguous associations resulted, and these precise positions were calculated far too late to look for related visible-light variations.
The 1996 launch of an Italian-Dutch satellite called BeppoSAX has since revolutionized the field. How so? First, BeppoSAX has established that an X-ray afterglow often follows within hours of a GRB. Since X-ray photons are"cooler" (less energetic) than gamma-ray ones, this provided the first evidence for an expanding, cooling fireball. Second, BeppoSAX's detectors can pinpoint X-ray afterglows to within a few arcminutes — a fiftyfold improvement in positional precision. As a result, astronomers could precisely direct ground-based telescopes to the sites of GRB's within hours.
European astronomers did just that when they pointed telescopes in Italy and on La Palma toward GRB 970228, a February 28, 1997, burst in Orion. And the images they acquired made astronomical history. They detected a fading "star" within the arcminute-wide patch of sky that contained the burst's X-ray afterglow. Deeper Hubble Space Telescope (HST) and Keck Observatory images then revealed an amorphous blob next to the fading point of light. Many astronomers take this blob to be the galaxy within which the burster exploded eons ago.
Skeptics pointed out that the variable "star" and the GRB could very well be unrelated phenomena that overlapped only by coincidence. But then, a second GRB counterpart was caught by the Kitt Peak and Palomar Mountain observers.
According to Chryssa Kouveliotou (Universities Space Research Association), GRB 970508, the May 8th burst in Camelopardalis, was relatively weak as GRBs go. But the variable "star" that brightened in its position in two days later gave off enough light for the enormous Keck II reflector to obtain a spectrum. And that spectrum seemed to show the spectral fingerprint of intervening gas at least 4 billion light-years away. This provided the strongest support yet for a "cosmological" GRB distance scale.
What's more, sensitive arrays of radio telescopes then
picked up flickering radio emission from precisely the same location
as the variable "star." The radio data even indirectly indicated that the
source was expanding nearly at light speed. To Princeton theorists Bohdan
Paczynski and Eli Waxman, this greatly strengthens the case for a compact
fireball that exploded with dozens or hundreds the times the force of a
supernova.
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However, it remains conjecture just how the explosion was set off, and in what setting. Unlike its February precursor, GRB 970508 shows no sign of a host galaxy. Ad five other well documented bursts have yielded no visible light counterparts whatsoever, despite equally vigilant follow-up. In fact, very deep searches of four sites with the HST have ruled out any host galaxies at all! Either the galaxies are at seemingly impossible distances, or they are rare, dim dwarfs in four out of four cases — or the bursts took place in intergalactic space where practically no stars exist. Each explanation seems implausible.
Fortunately, prospects for obtaining better clues have never been brighter. Although hobbled by gyroscope failures last summer, BeppoSAX resumed operation in September. The asteroid-chasing NEAR spacecraft will soon join GRO and Ulysses to form a long-baseline triangulation network. And the new High Energy Transient Explorer (HETE), slated for a 1999 launch, should provide precise and immediate positions for dozens of GRBs yearly.
Send me your comments or questions at the following address:
Joshua Roth, Courtesy Sky & Telescope Magazine , February 1998.
