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Midnight Burst Thrills Astronomers
9 May 2005 (All day)
Astronomers may have for the first time witnessed the colossal crash of two neutron stars on the fringes of a distant galaxy. A NASA satellite detected a sharp flare of energy just moments after 12 midnight EDT on 9 May, triggering hours of intense research by telescopes on the ground. Analysis is ongoing, but a neutron-star merger--if confirmed--would help explain a type of explosion that has puzzled researchers for years.
The blasts, called gamma ray bursts (GRBs), come in two varieties. A "long" burst, lasting from seconds to a few minutes, arises from a gigantic star that explodes when its dense core collapses and creates a black hole, according to the leading theory. But "short" bursts, emitting pulses of gamma rays in fractions of a second, are utterly mysterious. Astrophysicists hypothesize that some short GRBs should come from colliding neutron stars, the dense spinning remnants of supernovas that do not form black holes.
Now, a NASA satellite called Swift has caught the first short GRB witnessed directly by astronomers. The satellite picked up a spike of gamma rays lasting 1/20th of a second from the constellation Coma Berenices. In less than a minute, the satellite swiveled to point its x-ray telescope at the GRB. It detected 11 x-ray photons--an extremely faint signal, but enough to notify ground-based telescopes of the approximate location. As the night progressed, at least two telescopes--the 3.5-meter WIYN Telescope at Kitt Peak, Arizona, and the 10-meter Keck I Telescope at Mauna Kea, Hawaii--saw a faint patch of light within that search area, near a galaxy about 2.7 billion light-years away. The galaxy is a blob of ancient stars where no new stars have formed for billions of years.
The outskirts of such an old galaxy is exactly where astronomers expect to see neutron stars collide, says Swift lead scientist Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The scenario is this: Supernova explosions expel a pair of neutron stars out of their native galaxy and into deep space. Then, it takes billions of years for the pair to spiral together in a brief fury of energy, probably coalescing into a new black hole. "Everything seems to fit," Gehrels says. "It's the most interesting possibility for short bursts."
The excitement is warranted, says theorist Shri Kulkarni of the California Institute of Technology in Pasadena, California. Still, he warns, researchers have studied their images for just a few hours. They must confirm that the patch of light seen by their telescopes came from the burst itself and not from an unrelated object at a different distance.