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Gamma-ray bursts (GRBs) come in two flavors: short and long. The dividing point between long and short is about 2 seconds. The long GRBs have been studied for several years through very precise localizations of their positions, and we have learned that they tend to occur within the spiral arms of star forming galaxies, and are associated with the explosions of massive stars that create powerful jets that are beamed toward the Earth. |
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Until very recently the short GRBs remained mysterious, due to the lack of any localizations on the sky. In the past several months this situation has changed. Thanks to rapid responses from alerts by the HETE-2 and Swift spacecraft, astronomers have finally obtained very precise positions for a handful of short GRBs. These were bursts that occurred on 2005 May 5th, July 5th, and July 24th. | |
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The precise localizations showed two of these bursts occurring on the outskirts of older, elliptical galaxies, and one burst outside an irregular galaxy. That fact that the localizations place them outside a galaxy rather than inside it are consistent with the idea of neutron star-neutron star merger, in which the system receives a ``double kick'' from the supernova explosions that create each neutron star.
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After a period of several hundred million years, the energy carried out of the binary orbit due to gravitational radiation causes the two neutron stars to spiral inward and finally coalesce, in a fiery explosion producing a short GRB.
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The most famous example of a double neutron star binary within our
own galaxy is ``The Binary Pulsar'' discovered by Hulse & Taylor
in 1974,
for which they were awarded the
Nobel Prize in 1993. The orbit of this
binary is decaying and the two neutron stars will merge in 300 million
years.
The binary star system has to acquire a velocity of greater than about 300 km/s to achieve the speed necessary to escape the galaxy. By comparison our own solar system orbits the center of the Milky Way at about 200 km/s and the Earth orbits the Sun at 30 km/s. For More Info: See Gamma-Ray Bursts: Introduction to a Mystery [14+ yrs; Imagine the Universe!], Gamma ray burst [Wikipedia], Gamma-ray Bursts FAQ [by Edo Berger, Caltech], Gamma-ray Burst Real-time Sky Map [Sonoma State University], and (of course) Google "Gamma-ray bursts" "physics of". | ||
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John Cannizzo is an Associate Research Scientist member of the
Joint Center for Astrophysics,
a collaboration between the
Physics Department at
UMBC and
the
Exploration of the Universe Division
at the
NASA/Goddard Space Flight Center.
John spends most of his time at NASA/GSFC, and can be contacted via e-mail to
cannizzo@stars.gsfc.nasa.gov.
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