May 21, 1998
May confirm existence of 'magnetars'
HUNTSVILLE, Ala. (May 20, 1998) -- The most intense magnetic field ever found in the universe has been observed around a neutron star 40,000 light years from Earth, according to an international team of astronomers led by scientists working at NASA's Marshall Space Flight Center in Huntsville, Ala.
This discovery may confirm the existence of "magnetars," a special class of neutron stars with magnetic fields estimated to be one thousand trillion times the strength of Earth's magnetic field.
"The magnetic field generated by this star is truly incredible," said Dr. Chryssa Kouveliotou, the Universities Space Research Association scientist who led the team of astronomers. "It is so intense that it heats the star's surface to 18 million degrees Fahrenheit."
A report on the discovery will be published in the May 21 issue of the journal "Nature."
Kouveliotou's team included Dr. Stefan Dieters of the University of Alabama in Huntsville (UAH), Professor Jan van Paradijs of UAH and the University of Amsterdam, and Dr. Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, MD.
Using data from two satellites, Kouveliotou and her team determined the strength of the star's magnetic field by measuring the speed at which its rotation is slowing -- an effect believed to be caused by the star's powerful magnetic field.
The neutron star in question, designated SGR 1806-20, is observed to be spinning once every 7.5 seconds. The rate at which it spins is slowing by roughly three milliseconds per year. Superstrong magnetic fields would cause a neutron star to "brake" and "cool down," making it practically impossible to observe them in the radio wave and X-ray spectra at which neutron stars are usually observed.
That means there could be thousands -- perhaps millions -- of these dark relics scattered throughout our Milky Way galaxy, accounting for the large number of supernovae remnants which have no detectable neutron stars at their centers.
A neutron star is a burned-out star, roughly equal in mass to the Sun, that has collapsed through gravitational forces to be only about 10 miles across; magnetars have a magnetic field that is about 100 times stronger than the typical neutron star.
Astronomers believe that at least 10 percent of neutron stars are born with magnetic fields that are strong enough to be considered magnetars. Neutron stars are created in supernovae explosions and they spin rapidly, at rates up to hundreds of revolutions per second.
"This finding should help us better calculate the rate at which stars die and create the heavier elements that later become planets and other stars," Kouveliotou said.
The data Kouveliotou and her team used to measure the star's spin rate was gathered by NASA's Rossi X-Ray Timing Explorer satellite and by the Advanced Satellite for Cosmology and Astrophysics, a collaborative mission between Japan and the United States.
SGR 1806-20 belongs to a class of neutron stars called "soft gamma repeaters."
"Periodically, the magnetic field drifts through the crust of the neutron star, exerting such colossal forces that it causes a 'starquake,'" said Kouveliotou. "The 'starquake' energy is released as an intense burst of low-energy gamma rays."
These bursts happen quite often. When bursting, Soft Gamma Repeaters are among the brightest objects in the sky, giving off as much energy in a single second as the Sun does in an entire year. SGR 1806-20 was discovered when it emitted soft gamma ray bursts.
Astronomers have debated the origin of Soft Gamma Repeaters since they were first observed in 1979. With this discovery, however, researchers believe the origin of Soft Gamma Repeaters lies in the "starquake" phenomena of magnetars. The magnetar theory was proposed in 1992 by astrophysicists Dr. Robert Duncan of the University of Texas at Austin and Dr. Christopher Thompson of the University of North Carolina at Chapel Hill.
For more information on magnetars and this discovery, visit NASA Marshall's Space Sciences Laboratory website at:
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The above story is based on materials provided by University Of Alabama In Huntsville.
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