GAMMA-RAY BURST 080319B
Judith Racusin

A new study of a stellar explosion visible from halfway across the universe finds that the blast had an unusual structure that researchers heretofore had never observed.

Gamma-ray burst GRB 080319B was already on record as the brightest stellar explosion ever recorded.

Now astronomers have analyzed the visible afterglow of the burst and found that the emitted light peaked initially an hour after the burst and a second time 11 days later.

They report today in Nature that the most likely explanation is the GRB produced a pair of relativistic (very high speed) jets, one inside the other. Gas and dust surrounding the exploding star flared with light each time a jet struck it.

Detectors on NASA's Swift Gamma-Ray Burst satellite and by the Russian Konus gamma-ray instrument on board NASA's Wind satellite detected the initial burst on March 19.

It just so happened that a pair of optical telescopes on the ground was observing the last GRB, 080319A, which had gone off 30 minutes before in the same part of the sky.

These telescopes, along with Swift's own UV/Optical Telescope and other robotic telescopes alerted by the satellites, monitored the six-week afterglow of visible light following the burst.

Scientists believe GRB 080319B would have been visible to the naked eye for about 40 seconds if anyone had been looking its way, toward the constellation Bootes.

This is all the more remarkable given that it exploded an estimated 7.4 billion years ago—before the sun and Earth had formed.

It's the most complete picture of a GRB ever seen, says study author Judith Racusin, an astrophysicist at Pennsylvania State University in University Park.

"They've been sort of waiting around for something this bright," she says, referring to the telescopes. "It was just really luck that the brightest burst happened to occur and they were looking at it."

Longer lasting GRBs are believed to occur when a massive star collapses into a black hole, sparking a supernova explosion and whipping gas and dust into a pair of jets projecting in opposite directions.

Researchers had suggested that a nested jet could exist, Racusin says, but nobody had envisioned the inner jet moving so fast and producing all the gamma-rays by itself—part of the team's interpretation.

She notes that the strategy of using Swift to aim ground telescopes at GRBs is paying off, turning up rarer kinds of bursts that push beyond the simplest models for the explosions. "We're not going to see them very often," she says, "so we just need to keep looking."