A Cosmic Sequence Never Seen Before

An unusually long-lasting blast of radiation from a galaxy about 470 million light years away has given astronomers an unprecedented opportunity to view a supernova -- a cosmic explosion marking the collapse of a "supermassive star" -- from start to finish.

This spectacular cosmic sequence began Feb. 18 when NASA's Swift satellite detected a jet of radiation, known as a long-duration gamma ray burst, coming from a galaxy in the constellation Aries. Within minutes, astronomers throughout the world had trained telescopes on the spot.

Gamma ray bursts are the brightest and most powerful explosions in the universe, but the Feb. 18 explosion was unusual in that it was weaker but much closer to the sun than most detected bursts, which often occur in galaxies billions of light years away. The gamma rays from the burst lasted about 30 minutes -- 100 times longer than a typical burst.

"We've never seen anything like it," said Goddard Space Flight Center astrophysicist Frank Marshall, a member of the Swift science team.

Instead of fading immediately, the optical and ultraviolet light from the burst strengthened for more than two hours and 40 minutes and lingered at its most powerful level for more than a day.

Supernovae and gamma ray bursts are different manifestations of the same event -- the collapse and subsequent explosion of a supermassive star. Astronomers have been aware of supernovae since ancient times but have been able to detect gamma ray bursts only in the past 40 years.

Marshall said the energy from a gamma ray burst is hard to quantify because it is expelled in a single beam or "jet," but for this reason it is also regarded as significantly more powerful than the accompanying supernova, which dissipates its energy in all directions.

Though astrophysicists have long known that gamma ray bursts are tipoffs that stars have exploded, most bursts originate so far away that the supernovae cannot be detected.

The Feb. 18 burst, however, was close enough for the exploding star to be discerned through telescopes, and Swift enabled astronomers to focus on the event immediately. Several days passed before the burst's afterglow dissipated, making it possible for observers to optically confirm the supernova for the first time last weekend.

Marshall said optical telescopes should be able to see the supernova for "several weeks." Scientists in the Netherlands reported observing it with a 10-inch telescope, no bigger than many instruments used by amateurs. Marshall said, however, that observations would be easier under dark skies.

The Feb. 18 event was the second gamma ray burst to be followed by an observable supernova, but the earlier explosion, in 1998, was of far shorter duration, and it was not possible to bring telescopes to bear on the supernova for several days.

"People said then 'That's interesting, but unique,' " recalled astrophysicist Stanford E. Woosley of the University of California at Santa Cruz. "But now we've seen two, and they were so faint that the only reason we saw them was that they were nearby. This may suggest that these are the most common kind."

Or not. "Keep in mind, Swift is the first satellite to be able to swivel around and focus on the bursts as they occur," said Harvard's Robert P. Kirshner, president of the American Astronomical Society. "It's been up [since November 2004] and we've only seen one." The 1998 burst was detected by an earlier satellite.

One mystery posed by the gamma ray burst known as GRB 060218 is why it lasted so long. Marshall suggested that astronomers were seeing the burst not head-on, like a train's headlight, but at an angle. That would account for its faintness. But not necessarily for its long duration.

"It's not clear what's happened," Woosley said. "We need to know how energetic the gamma ray burst was initially. It wasn't that powerful to begin with, but for some reason the engine kept pumping energy."

Another mystery is why so few supernovae -- less than 1 percent -- are announced by gamma ray bursts. Woosley, a leading theorist in the study of supernovae, suggests that a burst is possible only if the exploding star is spinning.

Supernovae occur when supermassive stars -- at least eight times as large as the sun -- burn out and collapse upon themselves. If a star is bigger than 20 solar masses, scientists theorize, much of the matter will create a black hole, while some will be ejected in a massive explosion -- the supernova.

"But if you spin the star fast enough, matter hangs up in the hole," Woosley said in a telephone interview. "It gets very hot and generates a lot of energy," driving gamma ray jets out the mouth of the black hole.

The gamma ray burst and the supernova explosion occur at the same time, but the burst gets to the surface of the black hole first, Woosley said, and it is always observed before the supernova.