Satellite Uncovers Clues to Mysterious Cosmic Bursts

A new satellite may finally give astronomers the means to figure out what is causing the mysterious cosmic explosions that space-based instruments have detected for a quarter-century.

Since military satellites first discovered them in the 1960s, scientists have detected more than 2,000 gamma ray bursts. Yet they know almost nothing about why distant spots in the sky suddenly light up with high-energy gamma rays, only to fade to black in a few seconds or minutes.

The main obstacle has been logistical. After a burst lights up the sky, astronomers can’t turn the world’s most powerful telescopes on the spot fast enough. By the time the telescopes are aligned, whatever made the burst has faded to invisibility.

That all changed Feb. 28, when the Italian-Dutch satellite BeppoSAX spotted a gamma ray burst in the constellation Orion.

Within eight hours the satellite had trained its own X-ray telescope on the spot where the gamma rays appeared and saw a fading fireball right where the burst had been. In subsequent weeks, the Hubble Space Telescope and several powerful earthbound telescopes also set their sights on the spot.

What they saw there looked like a fading fireball in a far-off galaxy, a team of 31 astronomers wrote in the April 17 issue of the journal Nature.

“Now we finally have something we can study,” said Gerald Fishman, one of the authors of the Nature paper. “It’s probably one of the biggest developments to happen in gamma ray burst research in the last 20 years.”

BeppoSAX will have to catch more gamma ray bursts before it can help figure out where the bursts come from and what is making them. But astronomers are optimistic, because on April 2 the satellite measured X-rays in the aftermath of yet another burst.

“The next burst which comes up I’m sure will be very well observed,” said Bohdan Paczynski, an astrophysicist at Princeton University. “This is really a major breakthrough from BeppoSAX.”

Launched in 1996, the satellite was designed in part to determine the locations of gamma ray bursts as quickly as possible and notify astronomers on the ground. The satellite simply scans part of the sky for gamma ray bursts, and when it sees one it sends a message to astronomers around the world saying, “Look over there.”

The authors of the Nature paper responded to BeppoSAX on Feb. 28 by turning two telescopes in the Canary Islands on the spot indicated by BeppoSAX. The telescopes made out both the fading fireball and a faint blurry object that may be a galaxy, as did the Hubble Space Telescope and other ground-based instruments.

“We see a fuzzy nebulosity right next to or adjoining the optical transient, but it’s hard to say that it’s actually a galaxy,” said Fishman, a research scientist at NASA’s Marshall Space Flight Center in Huntsville, Ala.

Though previous satellites have done the same thing as BeppoSAX, they have been either too slow or too imprecise to be useful, Paczynski said. BeppoSAX can lead astronomers to gamma ray burst remnants because it combines speed and precision in a single package.

If the gamma ray bursts do come from distant galaxies, as the BeppoSAX observation suggests, that invalidates the large number of gamma ray burst theories that require sources within the Milky Way, said Martin Rees, Britain’s Astronomer Royal and a professor at Cambridge University.

But at this stage, it’s too early to tell.

“My feeling is we have to wait for another one to be sure,” Rees said.