Washington, September 20 : NASA's Swift satellite has found the most distant gamma-ray burst ever detected, designated GRB 080913, which arose from an exploding star 12.8 billion light-years away.
"This is the most amazing burst Swift has seen," said the mission's lead scientist Neil Gehrels at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It's coming to us from near the edge of the visible universe," he added.
Because light moves at finite speed, looking farther into the universe means looking back in time.
GRB 080913's "lookback time" reveals that the burst occurred less than 825 million years after the universe began.
The star that caused this "shot seen across the cosmos" died when the universe was less than one-seventh its present age.
"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.
Gamma rays from the far-off explosion triggered Swift's Burst Alert Telescope on September 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 the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, captured the bursts fading afterglow.
The telescope's software listens for alerts from Swift and automatically slewed to the burst position.
Then, the team's Gamma-Ray Burst Optical/Near-Infrared Detector, or GROND, simultaneously captured the waning light in seven wavelengths.
"Our first exposure began just one minute after the X-Ray Telescope started observing," Greiner said.
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 blasts redshift at 6.7 - among the most distant objects known.