Satellite spots farthest-out blast

The Swift satellite has found the most distant gamma-ray burst ever detected, unleashed by an exploding star 12.8 billion light-years away, NASA announced Friday.

"It's coming to us from near the edge of the visible universe," lead mission scientist Neil Gehrens of NASA's Goddard Space Flight Center said in a space agency statement about the blast.

Because light moves at finite speed, looking farther into the universe means looking back in time. The "lookback time" implies that the burst, designated GRB 080913, occurred less than 825 million years after the universe began.

"This burst accompanies the death of a star from one of the universe's early generations," said Patricia Schady of the Mullard Space Science Laboratory at University College London, who is organizing Swift observations of the event.

NASA said gamma rays from the far-off explosion triggered Swift's Burst Alert Telescope at 1:47 a.m. ET Sept. 13. The spacecraft established the event's location in the constellation Eridanus and quickly turned to examine the spot. Less than two minutes after the alert, Swift's X-Ray Telescope began observing the position. There, it found a fading, previously unknown X-ray source.

Astronomers on the ground followed up as well. Using a 2.2-meter telescope at the European Southern Observatory in La Silla, Chile, a group led by Jochen Greiner at Germany's Max Planck Institute for Extraterrestrial Physics captured the burst's fading afterglow — thanks to an automated system linked to Swift.

The ESO team's Gamma-Ray Burst Optical/Near-Infrared Detector, or GROND, recorded the waning light in seven wavelengths. "Our first exposure began just one minute after the X-Ray Telescope started observing," Greiner said.

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In certain colors, the brightness of a distant object shows a characteristic drop caused by intervening gas clouds. The farther away the object is, the longer the wavelength where this fade-out begins. GROND exploits this effect and gives astronomers a quick estimate of an explosion's shift toward the less energetic red end of the electromagnetic spectrum, or "redshift," which suggests its record-setting distance.

An hour and a half later, as part of Greiner's research, the Very Large Telescope at Paranal, Chile, targeted the afterglow. Analysis of the spectrum with Johan Fynbo of the University of Copenhagen established the blast's redshift at 6.7 — making it among the most distant objects ever observed.

Gamma-ray bursts are the universe's most luminous explosions. Most occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets — driven by processes not fully understood — punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates bright afterglows.

The previous record holder was a burst with a redshift of 6.29, which placed it 70 million light-years closer than GRB 080913. The higher the redshift, the farther the distance.

The Swift satellite, which is named after a small, nimble bird species, was launched in November 2004. This January, the spacecraft's instruments caught the first X-rays from a new supernova days before optical astronomers saw the exploding star. In March, the satellite detected the brightest gamma-ray burst ever recorded.

This report is based on information from NASA.

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