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NASA and Gemini Probe Mysterious Distant Explosion
01.08.08
 
Swift's Ultraviolet Optical Telescope captured this image of the afterglow of GRB 070714B, the farthest short gamma ray burst with a measured distance. Swift’s Ultraviolet/Optical Telescope captured this image of the afterglow of GRB 070714B, the farthest short gamma-ray burst with a measured distance. Credit: Credit: Swift Team/UVOT/NASA
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When space-based satellites team up with large telescopes on the ground, they can be a powerful one-two combo for astronomical discovery. This is exactly what happened when astronomers used the giant Gemini North telescope in Hawaii to follow-up a discovery made with NASA’s Swift satellite. They found an incredibly powerful explosion halfway across the observable universe.

Swift detected the explosion on July 14, 2007, as a bright flash of energy in the constellation Taurus. Because the explosion emits most of its energy in the form of gamma rays (an ultra-high-energy form of light), the explosion is called a gamma-ray burst, or GRB. And because the explosion only lasted for 3 seconds, it is known as a short GRB. Naturally enough, GRBs that last longer than 3 seconds are known as long GRBs.

Astronomers have gathered very strong evidence that long GRBs are caused when massive stars explode. But they don’t actually know for certain what causes short GRBs. Astronomers think that most short GRBs are caused when two very dense but dead stars called neutron stars collide and merge, forming a black hole. The violent collision probably releases gamma rays in two jets that shoot away in opposite directions.



First frame of the GRB strip Nobody knows how the short gamma-ray burst GRB 070714B was triggered, but a leading possibility is the inspiral and merger of two neutron stars, depicted in this artist rendition. Credit: NASA/Dana Berry
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After Swift detected the short GRB on July 14, a team of astronomers led by John Graham of the Johns Hopkins University in Baltimore, Md., aimed the Gemini North telescope at the location of the burst. The telescope revealed the galaxy where the GRB took place. By taking a spectrum of the galaxy and measuring its characteristics, they could determine the distance to the GRB: an astonishing 7.4 billion light-years. This means that the explosion occurred 7.4 billion years ago, which is before Earth even formed!

Astronomers have seen long GRBs at greater distances, but this is the most distant short GRB ever seen. "This discovery dramatically moves back the time at which we know short GRBs were exploding. The short burst is almost twice as far as the previous confirmed record holder," says Graham. Graham is presenting his group’s discovery on Tuesday, January 8, at the American Astronomical Society’s 2008 winter meeting in Austin, Texas.

"The fact that this short burst is so far away means this subclass has a broad range of distances, although they still tend to be closer on average than long GRBs," says Swift lead scientist Neil Gehrels of NASA’s Goddard Space Flight Center in Greenbelt, Md.

First frame of the GRB stripThese illustrations show the view from afar. In the third image, we see two jets shooting away from the collision site. These jets are where the gamma rays are emitted. Credit: NASA/Dana Berry
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Gehrels adds that the energy of the July 14 GRB is about 100 times higher than average for short bursts, and is more similar to the typical energy of a long GRB. "It is unclear whether another mechanism is needed to explain this explosion, such as a neutron star-black hole merger," he says.

"We now have a good idea of the type of star that produces the brighter long bursts. But how short bursts are formed remains a mystery," says team member Andrew Fruchter of the Space Telescope Science Institute in Baltimore, Md.

 
 
Robert Naeye
Goddard Space Flight Center