Published online 21 March 2008 | Nature | doi:10.1038/news.2008.687

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Powerful stellar blast sighted

Light from cosmic explosion traversed half the universe.

Scientists have discovered a record gamma-ray burst, a massive explosion that occurred halfway across the universe but was so bright that its optical flash was briefly visible to the naked eye.

The Swift satellite captured the X-ray glow (above) that followed the gamma ray outburst.NASA/Swift/Stefan Immler, et al.

Gamma-ray bursts are high-energy explosions seen as shockwaves jet out from a massive dying star. They occur somewhere in the sky about once a day. This particular blast went off at 2:12 a.m. Eastern time on Wednesday, triggering automatic detectors on NASA’s Swift satellite. Some 15 astronomers were already up on a conference call discussing an earlier burst when their pagers started beeping again.

“We at first thought something was wrong with the satellite,” says Swift principal investigator Neil Gehrels, of the Goddard Space Flight Center in Greenbelt, Maryland. “Then we realized it was the most remarkable burst we’ve ever seen.”

Across the universe

The blast, dubbed GRB 080319B, initially triggered one of Swift’s three instruments, the gamma-ray detector. Then came a flash in visible light, so bright that it saturated Swift’s optical telescope. Several hours later, the Very Large Telescope in Chile reported on the blast’s spectral lines, which allowed astronomers to calculate its distance. The burst, it turned out, lay 7.5 billion light-years from Earth — more than halfway across the universe.

The team calculated that, had the explosion had gone off in the centre of the Milky Way, it would have appeared as bright as the noon-time Sun. In addition to Swift, some 60 ground-based telescopes observed the burst, and many will continue to watch it as its afterglow lingers for days.

Insights expected

Analysis of the gamma-ray burst should help in determining the metal content of galaxies over time, says Gehrels. Stars, which begin with hydrogen and helium, are the factories that make heavier elements, dispersed after the stars blow up as supernovae. The gamma-ray bursts travel through these gas clouds, which selectively absorb certain wavelengths of the burst depending on their chemical composition. This bright burst will thus provide a very clear data point for the chemical composition of a 7.5-billion-year-old galaxy.

And Peter Mészáros, who heads the Swift theory team at Penn State in University Park, Pennsylvania, will get an intriguing case to help him ponder the processes during star death that lead to the bursts. More than a decade ago, Mészáros published important theories about the shockwaves that excite gamma rays — followed by X-rays and optical light — that shoot out in narrow jets from poles of exploding stars.

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He wants to understand what could have caused such a large optical flash during the 19 March burst, which he describes as “crazy”. Mészáros also has to figure out why, in general, so few optical flashes have been detected compared to a higher number of gamma ray bursts.

He’ll have some time to work on the problem without the distraction of a beeping pager. “They don’t call the theorists at 2 a.m., thank God.” 

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