Works in Progress

Gamma-ray bursts illuminate the farthest reaches of space

By Karen Wright|Thursday, August 01, 2002
RELATED TAGS: STARS


HETE is flanked by solar panels, and its gamma-ray detectors, built into the top, face away from the sun. The satellite is also equipped to detect and pinpoint X rays.
Photograph courtesy of MIT
Ground-based telescopes paint a relatively tame picture of the cosmos. Earth's atmosphere and ionosphere screen out all forms of radiation except visible light, radio waves, and infrared signals. So, until the advent of satellites half a century ago, astronomers never captured the high-frequency wavelengths that issue from the most energetic explosions in space. "We were peeking out of this tiny little crack in the spectrum," says astrophysicist Donald Lamb of the University of Chicago. "Then, in the space age, the door swung wide open, and we went, 'Wow.'"

Space-based instruments beamed down scenes of extraordinary violence. Ultraviolet light streamed from dense, dying stars. X rays marked black holes and solar flares. And on July 2, 1967, two U.S. defense satellites on the lookout for clandestine nuclear testing detected a far more explosive phenomenon coming not from the Soviets but from some unidentified celestial source: a burst of gamma rays. The most powerful form of radiation in the electromagnetic spectrum, gamma rays were known to stream diffusely from radioactive nuclei and disturbances in the sun's magnetic field. But no one expected to see bright, brief gamma-ray beacons from the edges of the universe. The singular bursts fired more photons than any star in the firmament. They started suddenly, lasted only a few seconds, and then just as abruptly disappeared.

Scientists have been trying to make sense of gamma-ray bursts ever since. "They're the most incredible astronomical events we know about," says Lamb. More than 100 models have been proposed to explain the origin of the phenomena. Some rely on mechanisms as far-fetched as comet-anticomet annihilation and interstellar warfare. But observations collected during the past decade support more-promising theories. Today most astronomers think gamma-ray bursts result when a massive and rapidly rotating star collapses in on itself to form a black hole. The collapsing star, or collapsar, is a kind of supernova, but the extraordinary heft and spin of the star bring it to a faster and more calamitous end.

Computerized simulations suggest that the tremendous heat of the star's implosion gets funneled into two jets of matter and energy. One jet shoots out above the spiraling plane of the collapse; the other shoots out below it. As the dual plumes move at velocities near the speed of light, energy and debris in the plumes collide, prompting a kind of shock effect—an instantaneous transfer of energy that creates a burst of gamma radiation. While the average supernova may burn for weeks or months, a collapsar's energy is compressed into a single burst that typically lasts 20 seconds. "A gamma-ray burst is more than 10 million times brighter than an ordinary supernova," says Stan Woosley of the University of California at Santa Cruz, an author of the collapsar theory.

Gamma-ray bursts are so short-lived, astronomers have had few clues to their origin. But images from the Hubble Space Telescope support the idea that they occur during the collapse and death of a massive star. After a gamma-ray burst in 2001, the Hubble Space Telescope scanned the galaxy near where the burst occurred. Over a five-month period, it captured the remnants of the gamma-ray burst's afterglow.
Photograph courtesy of Shri Kulkarni/Caltech (5)
In the early 1990s, NASA's Compton Gamma Ray Observatory logged about 400 burst sightings a year. But the instrument couldn't locate a burst's position in space. Finding the source of a gamma-ray burst is like trying to figure out who threw a peanut at your head at a ball game. There's no warning it's going to happen, there are plenty of suspects, and the impact gives only vague clues to the object's trajectory. Among the thousands of bursts observed since their discovery, the provenance of only a handful is known. And it's hard to say what a gamma-ray burst is if you can't tell where it is.

That impasse began to ease in 1997, when an Italian-Dutch satellite called BeppoSAX spotted a burst afterglow—a fading remnant of X rays, visible light, and radio waves that can linger for days and even months after the gamma rays are gone. Afterglows are the Rosetta Stone of gamma-ray astronomy. Because they last so much longer than bursts, they give astronomers time to calculate the distance and direction of a source. Afterglow studies have confirmed that, in addition to being the most luminous and energetically intense events ever recorded, gamma-ray bursts are also the most distant. The farthest one located so far occurred nearly 12 billion light-years away; its radiation was liberated just before the birth of the Milky Way galaxy.

But astronomers think the bursts and their afterglows are in principle bright enough to reach even farther across space and time. They could shine from an era when the cosmos emerged from the few hundred million years of darkness that followed the Big Bang some 15 billion years ago. The afterglows would carry signatures of the conditions that prevailed in the youngest stars as they forged the elements that would build the rest of the universe. "You could see the moment of first light, the formation of the first stars, and the making of heavy elements," says Lamb. And as the fading light of an afterglow travels toward Earth, it picks up the elemental signatures of every galaxy and gas cloud between here and there—a map of the structure and composition of space across time.

The great distance of gamma-ray bursts makes sense because the heavy, spinning stars that are thought to fuel them were probably more plentiful in the early universe. Last April an international team of astrophysicists presented the strongest evidence yet that a specific burst did in fact accompany the demise of a massive star. But the connection between gamma-ray bursts and supernovas is still far from clear. And it's gotten more complicated with the help of High Energy Transient Explorer, or HETE, a NASA satellite launched in 2000. HETE (rhymes with Betty) is the most sensitive gamma-ray instrument ever to roam the skies. It can calculate a burst's position within seconds of detection and beam the coordinates to ground stations that are strung like pearls on a necklace around the equator. The stations then send a burst alert to a Massachusetts Institute of Technology control center, which distributes it to a global network of gamma-ray aficionados. Within minutes, ground- and space-based telescopes can extend HETE's observations to other wavelengths.

So far, the satellite and its crew have bagged 17 clear-cut burst events. But some of the fireworks don't fit the predicted profiles. A fifth of all observed bursts, for example, are too short to be explained by the collapsar theory. Lasting less than a second, these super-short gamma-ray bursts pack more of a wallop than the longer ones. HETE and other space-borne instruments have also seen brief hybrid flashes composed of both X rays and gamma rays. And afterglows are turning out to be quite idiosyncratic. Some radiate only in visible light, others only in X rays and radio waves.

"When we see these things, we scratch our heads and say, 'This is a weird one—wonder what we should do with it,'" laments George Ricker of MIT's Center for Space Research, who is head of the HETE team.

Three more gamma-ray missions are slated in the next decade to help sort things out. In the meantime, HETE's data seem to have launched another round of speculation about the origins of the brightest sparks in space. Even the father of the collapsar model concedes that no one source could possibly explain the menagerie of new gamma-ray observations. "It's something of a zoo," says Woosley.







To view updated gamma-ray-burst information, see MIT's HETE site: space.mit.edu/HETE.


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