NASA and Gemini Probe Mysterious Explosion in the Distant Past (1/14/2008)

Using the powerful one-two combo of NASA's Swift satellite and the Gemini Observatory, astronomers 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 of the Johns Hopkins University, in Baltimore, Md. Graham is presenting his group's discovery in a poster at the American Astronomical Society's 2008 winter meeting 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 longer or shorter than three seconds.

Astronomers think that long GRBs are triggered by the collapse and explosion of massive stars. In contrast, a variety of mechanisms have 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 counterflowing beams.

Professor Nial Tanvir of the University of Leicester was part of the team responsible for this work. In addition, the precise X-ray position of this burst was provided by virtue of work done at Leicester.

Professor Tanvir said: "It is important to point sensitive telescopes at these bursts because they fade away so very quickly. Our measurement of its distance enables us to say just how violent the explosion was."

The record-setting short burst is known as GRB 070714B, since it was the second GRB detected on July 14, 2007. Swift discovered the GRB in the constellation Taurus. The burst's high energy and 3-second duration firmly place it in the short GRB category. 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 in Baltimore, and University of Leicester graduate Andrew Levan , of the University of Warwick, trained the 8-meter Gemini North Telescope in Hawaii on the galaxy. It revealed that the host galaxy has a spectral line from ionized oxygen. The amount that line was shifted toward the red end of the spectrum yields a redshift of 0.92. A redshift of 0.92 translates to a distance of 7.4 billion light-years, 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 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. 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 one of the beams happened to be aimed directly at Earth. This would make the burst seem more powerful than it really was.

Perhaps most short GRBs eject their energy in wider and 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," says Fruchter.

Swift is managed by NASA Goddard and was built and is operated in collaboration with Penn State University, the Los Alamos National Laboratory, and General Dynamics in the U.S.; the University of Leicester and Mullard Space Sciences Laboratory in the U.K.; Brera Observatory and the Italian Space Agency in Italy; plus partners in Germany and Japan.

Swift

Swift, a mission with substantial UK and Italian participation, is designed to solve the mystery of the origin of gamma-ray bursts. Scientists believe the bursts are related to the formation of black holes throughout the universe - the birth cries of black holes.

To track these mysterious bursts, Swift carries a suite of three instruments. The Burst Alert Telescope (BAT) instrument, built by NASA's Goddard Space Flight Center, detects and locates 2-3 gamma-ray bursts weekly, relaying a rough position to the ground within 20 seconds. The satellite automatically and swiftly re-points itself to bring the burst area into the narrower fields of view of the on-board X-ray Telescope (XRT) and the UltraViolet/Optical Telescope (UVOT). These telescopes study the afterglow of the burst produced by the cooling material that remain from the original explosion.

The XRT (with its Leicester-provided X-ray camera) uniquely finds a precise arc-second position of the burst. The XRT and the UVOT (with the MSSL-provided telescope) measures the spectrum of its afterglow in X-ray and visible wavelengths. For most of the bursts detected, Swift data, combined with complementary observations conducted with ground-based telescopes, enables the distances to the burst sources to be measured.

The afterglow phenomenon can linger in X-ray light, optical light, and radio waves for hours to weeks, providing detailed information about the burst. Swift routinely checks every discoverd burst regularly to study the fading afterglow, often working in conjunction with ground-based optical and radio telescopes.

Swift notifies the astronomical community via the Goddard-maintained Gamma-ray Burst Coordinates Network whenever a new burst is discovered, providing the crucial link of the burst's precise location directly other telescopes. The Swift Mission Operations Center, operated from the Pennsylvania State University campus, controls the Swift observatory and provides continuous burst information. Goddard manages Swift.

Swift was built through collaboration with national laboratories, universities and international partners, including General Dynamics, Gilbert, Arizona; Penn State University; Los Alamos National Laboratory, New Mexico; Sonoma State University, Rohnert Park, Calif.; Mullard Space Science Laboratory in Dorking, Surrey, England; the University of Leicester, England; ASI-Malindi ground station in Africa; the ASI Science Data Center in Italy; and the Brera Observatory in Milan, Italy.

Note: This story has been adapted from a news release issued by the University of Leicester

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