When a shot is fired, one expects to see a person with a gun. In the same way, whenever a giant star explodes, astronomers expect to see a galaxy of stars surrounding the site of the blast. This comes right out of basic astronomy, since almost all stars in our universe belong to galaxies.
But a stellar explosion seen last January has shocked astronomers because when they looked for the star’s parent galaxy, they saw nothing at all. The explosion took place in the middle of nowhere, far away from any detectable galaxy. The astronomers saw no hint of a galaxy even though they looked for one with the world’s largest telescope: the giant Keck I telescope in Hawaii.
"Here we have this very bright burst, yet it's surrounded by darkness on all sides," says Brad Cenko, an astronomer at the California Institute of Technology (Caltech) in Pasadena, Calif. Cenko is the leader of the team that made this discovery. The team includes astronomers from both Caltech and Penn State University.
The explosion belongs to a class of events know as gamma-ray bursts, or GRBs for short. GRBs are triggered when a very heavy star can no longer produce energy. The core of the star implodes to form a black hole — a region of space where gravity is so strong that nothing, not even light, can escape. The black hole spins very fast, producing intense magnetic fields. As inrushing gas from the star spirals toward the black hole, the magnetic fields fling some of the material away from the black hole in two powerful jets. These jets produce the GRB.
Several spacecraft detected the explosion on January 25, 2007. Observations by NASA's Swift satellite 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 large telescopes on the ground.
Using the team's robotic 60-inch telescope at Palomar Observatory in Calif., the astronomers discovered that the burst had a bright afterglow that was fading fast. They observed the afterglow in detail with two of the world's largest telescopes, the Gemini North telescope and the 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 and Keck observations saw no trace of a galaxy at the burst’s location. "A Keck image could have revealed a very small, faint galaxy at that distance," says team member Derek Fox of Penn State.
So why didn’t the team see a galaxy? One possibility is that the star formed in the outskirts of two galaxies that are colliding. Hubble Space Telescope images of colliding galaxies show that many of them have long star tails that are produced by the gravity of the two galaxies. These tails are very faint, and would not show up in Keck images at the burst’s measured distance from Earth. If this idea is correct, it should be possible to detect the tail by taking a long exposure with Hubble. "That's definitely our next stop," says Cenko.
"Many Swift discoveries have left astronomers scratching their heads in befuddlement," adds Swift lead scientist Neil Gehrels of NASA Goddard Space Flight Center in Greenbelt, Md. "But this discovery of a long GRB with no host galaxy is one of the most perplexing of all."
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