Getting a peek at power behind gamma bursts

by Usha Lee McFarling

Space: With help from the Hubble telescope, scientists learn that exploding stars cause the brilliant flashes of light.p. Since their accidental discovery by Cold War satellites monitoring for nuclear tests in space 35 years ago, gamma ray bursts have been among the most mysterious and extreme phenomena in the universe. No one has been able to say just what monstrous objects trigger the spectacular spasms of light, which burn for just seconds but with the brilliance of 10 billion suns.

The theoriessome serious, others offbeathave been nearly as numerous as gamma ray burst sightings themselves: rotating black holes, merging neutron stars, massive hyper-novas, alien spacecraft, an entirely new kind of physics or nuclear explosions with perfect efficiency. At least 140 theories have been advanced to explain these flashes of light that can shine as brightly as all the stars in the universe combined.

On Thursday, two teams of astronomers, one at Caltech and one at Notre Dame , announced that the Hubble Space Telescopeand follow-up with telescopes on Earthallowed them to find the culprit behind these cosmic fireworks. They are death flares sent out by supernovas, or exploding stars. “We’ve now got a very good idea of what the monster is underneath. It’s a massive collapsing star,” said Dale Frail, a radio astronomer at the National Radio Astronomy Observatory in Socorro, N.M., who is part of the Caltech team.

“They’ve got a smoking gun here,” said Jay Norris, an expert on gamma ray bursts at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Evidence Had Pointed to the Stars

Norris said recent evidence has been leading astronomers toward the idea that exploding stars probably cause the gamma ray bursts. Among the discoveries was an April announcement by University of Leicester astronomer James Reeves that the fireball-like afterglow of a burst contained the same chemical elements created by a supernova. Other observations show that the bursts originate in galaxies where stars are hatching and dying at high rates.

Until now, though, no one had made an observation crisp and well-defined enough to prove it.

“There have been a lot of what I call cocktail party suggestions” about the origin of gamma ray bursts, said John Bahcall, an astrophysicist at Princeton’s Institute for Advanced Studies. “A solid detection is a real breakthrough. It will transform the field.”

Making the observationsseeking the faint remnants of dying stars from across the universeproved extremely difficult. The success is another example of astronomers using a new generation of powerful telescopes to make observations that seemed impossible until recently.

“We’ve struggled for five years and only seen hints. Now it’s clear. It’s a clean signature,” said Shri Kulkarni, the Caltech astronomer who led the team.

Kris Stanek, an astronomer at the Harvard-Smithsonian Center for Astrophysics who helped lead the Notre Dame team with astronomer Peter Garnavich, called hunting for the source of the bursts “a detective story as challenging as any faced by the famous Lt. Columbo.”

The two teams made their observations Nov. 21 of a gamma ray burst that occurred about 4 billion to 5 billion light-years from Earth. They used the Hubble and several optical and radio telescopes in the Southern Hemisphere. Using the Hubble, the team detected the remnants of an exploded star, showing that a supernova accompanied the gamma ray burst. They sighted a distinctive fingerprint of a supernova—a “bump” in the light output that peaked a few weeks after the burst.

“They’re pretty good bumps,” Norris said of the new data.

The radio telescope observations helped confirm the theory by showing the explosion took place in a cocoon of hot gas that could only have been created by the massive star before it exploded. This helps disprove the other leading theory: that the bursts come from a collision of neutron stars, which are collapsed stars so dense they pack the mass of a star the size of the sun into an area the size of Chicago. The crash of neutron stars would not have left such a mess.

“Once it became clear that we had not only seen the supernova, but also the cocoon, I was very happy; I couldn’t sleep for days,” said Paul Price, a graduate student at the Australian National University and part of the Caltech team.

Astronomers and theorists struggled for years to understand how any object could send out such extreme amounts of energy. For years, neutron star or black hole mergers were thought to be the only things capable of expelling huge amounts of matter and energy at nearly the speed of light. Theories ranged so widely because astronomers didn’t know where the gamma ray bursts came from and whether they were originated within our own galaxy, even in our own solar system, or at the distant edges of the universe.

In 1997, Kulkarni led a Caltech team that used the Keck telescope in Hawaii to determine that many of the objects were at “cosmological distances”—so far away that they were created in the very earliest epochs of the universe. The most distant burst recorded so far occurred 30 billion light-years from Earth, Frail said. If the bursts can be seen across huge distances, Kulkarni said, “it really makes them the brightest objects in the universe.”

Norris also has discovered a class of gamma ray bursts that may originate relatively nearby, just a few hundred million light-years from Earth. These could also be caused by collapsing stars.

500 May Go Unnoticed Each Day

Other work on gamma ray bursts has shown that, while they are bright, they are not as energetic as once believed. The energy from the explosions is focused in narrow jets and is not expelled in all directions, which would require more energy. When the beams of light are pointed directly at Earth, they are very luminous. But many gamma ray bursts that occur in the universe, perhaps as many as 500 a day, are not pointed toward us and go undetected. There’s still plenty of work ahead. There is an entirely different class of gamma ray burstone that lasts less than two seconds and may be created by black hole or neutron star crashes. The new supernova explanation applies only to long-duration bursts, which make up the majority of the several thousand gamma ray bursts that have been recorded.

For the short-duration gamma ray bursts, there still is no explanation. “We know nothing about them,” Kulkarni said. “They remain completely mysterious.”

May 17, 2002

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