|
A team of astronomers at Penn State and Caltech has discovered a cosmic explosion that seems to have come from the middle of nowhere -- thousands of light-years from the nearest galaxy-sized collection of stars, gas, and dust. This "shot in the dark" is surprising because the type of explosion, a long-duration gamma-ray burst (GRB), is thought to be powered by the death of a massive star.
"Here we have this very bright burst, yet it's surrounded by darkness on all sides," says Brad Cenko of the California Institute of Technology in Pasadena, California, lead author of the team's paper, which has been accepted for publication in the Astrophysical Journal. "The nearest galaxy is more than 88,000 light-years away, and there's almost no gas lying between the burst and Earth."
The blast was detected on 25 January 2007 by several spacecraft of the Inter-Planetary Network. Observations by NASA's Swift satellite, controlled from its Mission Operation Center at Penn State, pinpointed the explosion, named GRB 070125 for its detection date, to a region of sky in the constellation Gemini. It was one of the brightest bursts of the year, and the Caltech/Penn State team moved quickly to observe the burst's location with ground-based telescopes.
Using the team's robotic 60-inch telescope at Palomar Observatory in California, the astronomers discovered that the burst had a bright and fast-fading afterglow in visible light. This discovery prompted them to observe the afterglow in detail with two of the world's largest telescopes, the 8-meter Gemini North telescope and the 10-meter Keck I telescope, both near the summit of Hawaii's Mauna Kea.
What came next was a total surprise. Contrary to experience with more than a hundred previous GRBs, the Gemini spectra revealed no signs of dense gas and dust absorbing the light of the afterglow. A trace of magnesium revealed that the burst took place more than 9.4 billion years ago, as deduced by the shift in wavelength of the afterglow's light, and that the surrounding gas and dust was more tenuous than the environment around any previous burst.
To further pin down the environment that could produce such an unusual explosion, the group obtained Keck images of the location of GRB 070125 long after its afterglow light had faded away. Surprisingly, the resulting images showed no galaxy at this location. "A Keck image could have revealed a very small, faint galaxy at that distance," says coauthor Derek Fox of Penn State.
|