Two years ago, nasa’s Marshall Space Flight Center in Huntsville, Alabama, hosted a conference on the astronomical phenomenon known as gamma-ray bursts. Plenty of observational astronomers showed up, but, says Gerald Fishman, a Marshall astrophysicist, the theorists mostly stayed away. It wasn’t for lack of interest: since their discovery in the late 1960s, these brief flashes of high-energy electromagnetic radiation have been one of the enduring mysteries of astrophysics. Explaining them would qualify as a major coup.
The trouble was, nobody could figure out where the gamma-ray bursts were—in and around our Milky Way galaxy or at the far reaches of the universe. That put the theorists in the position of trying to explain something that might be either a distant mountain or a nearby molehill. No wonder they didn’t make the trip. Things were rather muddled, admits Fishman.
This September, though, when astronomers reconvened in Huntsville for a follow-up meeting, the theorists came in droves. Thanks to a series of dramatic discoveries, it was suddenly and inescapably clear that gamma-ray bursts almost certainly take place deep in the cosmos, not nearby. And that meant they were not just moderately energetic blasts but rather some of the most powerful explosions in the universe.
For a long time after gamma-ray bursts were discovered—accidentally, by Defense Department satellites looking for Soviet nuclear detonations in space—astronomers knew next to nothing about them. In the mid-1980s, though, a Princeton astronomer named Bohdan Paczy´nski pointed out that they seemed to be coming from all directions in the sky, in which case their sources must be evenly distributed, too. The only objects that fit that bill are comets at the edge of the solar system, in the so-called Oort cloud, and galaxies far out in the universe. Comets being way too cold to emit gamma rays, Paczy´nski advocated the latter hypothesis—that the bursts came from far away, which meant their energy must be preposterously large. At first, recalls Fishman, Bohdan was the only one who took that position. So few bursts had been observed at the time that their distribution was still debatable.
In 1991, when the Compton Gamma Ray Observatory satellite went up and started recording bursts at the rate of one a day, it became clear that Paczy´nski had been right about their even distribution. But the clinching evidence for his hypothesis came only this past year, from an Italian-Dutch satellite known as Bepposax. Unlike all its predecessors, it has the ability to pinpoint the position of a burst—and on February 28, for the first time, it located one in a patch of sky about one-sixth the diameter of the full moon. Optical telescopes swung into operation and detected a fading spot of ordinary light at the position of the gamma-ray burst. A few weeks later both the Hubble telescope and ground-based telescopes found a faint visible glow precisely where the now-vanished burst had been. It looked like the ordinary glow of a distant galaxy—the one, presumably, that had emitted the sudden burst.
On May 8, Bepposax pinpointed a second burst that nailed the case shut. This time the world’s most powerful optical telescope, the Keck telescope in Hawaii, was able to spread the visible-light afterglow into a spectrum. Superimposed on the spectrum were dark absorption lines, caused by gases in a cloud somewhere between the burst and Earth—about 7 billion light-years from Earth, according to astronomers at Caltech. That convinced pretty much everyone in the bursts are nearby camp that the explosions—or at least some of them—were actually very distant.
Meanwhile, astronomers at the Very Large Array radio telescope in New Mexico were detecting the burst’s radio-wave aftermath, another first. The radio emissions flickered dramatically at the outset, apparently because of the shifting gases they were passing through—just as starlight twinkles from its passage through Earth’s atmosphere. But later the flickering stopped. That suggested the source of the burst had grown larger—the analogy would be a planet, which looks larger in the sky than a star and doesn’t twinkle as much as stars. Combined with the fact that bursts seem to evolve from energetic gamma rays to X-rays to visible light, which means they cool off over time, the radio data supported the idea that they are huge fireballs, expanding at near-light-speed and cooling as they go.
The ultimate cause of the fireball, though, is still a deep mystery. So far the leading candidates are the merger of two neutron stars and the swallowing of a neutron star by a black hole. For his part, Paczy´nski favors a sort of super-energetic supernova explosion. But it’s still too early to tell. These discoveries by Bepposax were a great breakthrough, says Paczy´nski, but we badly need more data. Over the next few years, satellites should provide them
