Swift sees most distant gamma-ray burst
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 23, 2008

NASA's Swift satellite has captured a far-flung gamma-ray burst from an exploding star located 12.8 billion light-years away, 70 million light years more distant than the previous record holder.

Gamma-ray bursts (GRB) are the Universe's most powerful explosions, occurring when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets punch through the star’s expelled shells and blast into space, striking gas previously shed by the star and heating it up, which generates bright afterglows that telescopes can detect.

View of the gamma-ray burst as seen by Swift's Ultra Violet and Optical Telescope, merged with the view through the X-ray telescope. Image: NASA/Swift/Stefan Immler.

"This is the most amazing burst Swift has seen," says Swift mission lead scientist Neil Gehrels. "It's coming to us from near the edge of the visible Universe." The burst 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," says Patricia Schady of the Mullard Space Science Laboratory at University College London, who is organising 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 13 September. The telescope established the GRB's position in the constellation Eridanus and quickly slewed to examine the scence. Less than two minutes after the alert, Swift's X-Ray Telescope began observing the position where it spotted a fading, previously unseen X-ray source.

GRB 080913 can't be seen in one of GROND's optical filters (left), but appears in another (right). The sudden appearance of objects at longer wavelengths indicates great distance. Image: MPE/GROND.

The observations were followed up from the ground, using telescopes such as a 2.2 metre telescope at the European Southern Observatory in La Silla, Chile. The Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) simultaneously captured the waning light of the GRB in seven wavelengths, with the first exposure taken just one minute after the X-Ray Telescope started observing.

In certain colours, 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 can study this effect, giving astronomers a quick estimate of an explosion's shift toward the less energetic red end of the electromagnetic spectrum - that is, its redshift - which enabled astronomers to determine the GRB’s record-setting distance.

Swift began its observing campaign in 2004, and in this year alone has detected both the brightest gamma-ray burst, which was even visible to the human eye, as well as the most distant GRB. It has also captured the first X-rays from a new supernova days before optical astronomers could see the exploding star.

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