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International Astronomers Team Has detected a Mysterious Type of Cosmic Explosion Farther Back in Time Than Ever Before
:: 10 January, 2008
Using the powerful one-two combo of NASA's Swift satellite and the Gemini Observatory, astronomers from a number of institutions, including Johns Hopkins, have detected a mysterious type of cosmic explosion farther back in time than ever before.
The explosion, known as a short gamma-ray burst (GRB), took place 7.4 billion years ago, more than halfway back to the Big Bang.
"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 John Graham, a graduate student in the Henry A. Rowland Department of Physics and Astronomy at The Johns Hopkins University. Graham is presenting his group's discovery at the American Astronomical Society's 2008 winter meeting this week in Austin, Texas.
GRBs are among the most powerful explosions in the universe, releasing enormous amounts of energy in the form of X-rays and gamma rays. Most bursts fall in one of two categories: long bursts and short bursts, depending on whether they last more or less than three seconds. Astronomers believe that long GRBs are triggered by the collapse and explosion of massive stars. In contrast, a variety of mechanisms has been proposed for short bursts. The most popular model says that most short GRBs occur when two neutron stars smash into each other and collapse into a black hole, ejecting energy in two counter-flowing beams.
The record-setting short burst is known as GRB 070714B, named because it was the second GRB detected on July 14, 2007. NASA's Swift Gamma-Ray Burst Mission discovered the GRB in the constellation Taurus. Rapid follow-up observations with the 2-meter Liverpool Telescope and the 4-meter William Herschel Telescope found an optical afterglow in the same location as the burst, which allowed astronomers to identify the GRB's host galaxy.
Next, Graham and his colleagues, Andrew Fruchter of the Space Telescope Science Institute and Andrew Levan of the University of Warwick in the United Kingdon, trained the 8-meter Gemini North Telescope in Hawaii on the galaxy. Analysis of a spectrum of the light from that galaxy indicated that it is 7.4 billion light-years away, meaning the explosion occurred 7.4 billion years ago.
"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.
Gehrels adds that GRB 070714B's energy was about 100 times higher than average for short bursts, 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," Gehrels said. "Or it could be that there are a wide range of energies for neutron star-neutron star mergers, but that seems unlikely."
Another possibility is that GRB 070714B concentrated its energy in two very narrow beams and that one of the beams happened to be aimed directly at Earth, making the burst seem more powerful than it really was. Researchers wonder if most short GRBs eject their energy in wider, less concentrated beams.
"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," Fruchter said.
Swift is managed by NASA's Goddard Space Flight Center and was built and is operated in collaboration with Penn State University, the Los Alamos National Laboratory, and General Dynamics in the United States; the University of Leicester and Mullard Space Sciences Laboratory in the United Kingdom; Brera Observatory and the Italian Space Agency in Italy; and partners in Germany and Japan.
About Swift Gamma-Ray Burst Mission
The Swift Gamma-Ray Burst Mission consists of a robotic spacecraft called Swift, which was launched into orbit on November 20, 2004, at 17:16:00 UTC (12:16 PM, EST) by a Delta II 7320-10C expendable launch vehicle. Swift is managed by the NASA Goddard Space Flight Center.
Swift is a multi-wavelength space-based observatory dedicated to the study of gamma-ray burst (GRB) science. Its three instruments work together to observe GRBs and their afterglows in the gamma-ray, X-ray, ultraviolet, and optical wavebands. This mission is part of NASA's "Medium Explorer Program" (MIDEX), and the satellite was developed by an international consortium from the United States, United Kingdom, and Italy.
About Gemini Observatory
The Gemini Observatory is an astronomical observatory consisting of two 8-metre telescopes at different sites. The Gemini telescopes were built and are operated by a consortium consisting of the United States, United Kingdom, Canada, Chile, Brazil, Argentina, and Australia. This partnership is managed by the Association of Universities for Research in Astronomy (AURA). The Northern Operations Center is located in Hilo, Hawaii, and the Southern Operations Center is in La Serena, Chile.
One telescope (Gemini North, also called the Frederick C. Gillett telescope) is located on Hawaii's Mauna Kea. Its location makes for excellent viewing conditions due to the superb atmospheric conditions on top of the over 4200 m (13,800 ft) high dormant volcano. It saw first light in 1999 and began scientific operations in 2000.
The other (Gemini South) is located at over 2700 m (9000 ft) elevation on a mountain in the Chilean Andes called Cerro Pachón. Very dry air and negligible cloud cover make this another prime telescope location (shared by several other observatories, including the SOAR Telescope and Cerro Tololo Inter-American Observatory). Gemini South saw first light in 2000.
