September 23, 2008 - NASA's Swift satellite has found the most distant gamma-ray burst ever detected. The blast, designated GRB 080913, 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. "It's coming to us from near the edge of the visible universe."
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.
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 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," says 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 at 1:47 a.m. EDT on 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 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. 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.
Analysis of the spectrum established the blast’s redshift at 6.7. The previous record holder was a burst with a redshift of 6.29, which placed it 70 million light-years closer than GRB 080913.
GRB 080913 exploded Sept. 13 at a distance of 12.8 billion light-years away in the constellation Eridanus. The box indicates the sky area shown in the Swift image.
This image merges the view through Swift's UltraViolet and Optical Telescope, which shows bright stars, and its X-ray Telescope, which captures the burst (orange and yellow). Credit: NASA/Swift/Stefan Immler