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MAGNETARS THE MOST MAGNETIC STARS IN THE UNIVERSE
Ultra-powerful magnetic neutron stars play hide-and-seek with astronomers. They're known to erupt without warning, some for hours and others for months, before dimming and disappearing again.



Above is an artist's rendering of an erupting neutron star. It can generate the most intense magnetic field observed in the Universe. The field strength of a magnetar is one thousand trillion times stronger than Earth's and is so intense that it heats the surface to 18 million degrees Fahrenheit. "We only know of about 10 magnetars in the Milky Way galaxy." remarked Dr. Peter Woods of the Universities Space Research Association. "If the antics of the magnetar we are studying now are typical, then there very well could be hundreds more out there." NASA research has suggested there may be far more magnetars than previously thought.
Observing the explosions from these celestial bodies has been tricky. The answer lies in the timing. So how do the researchers observe what has never been seen? Leave it to NASA to develop the perfect piece of equipment to handle the job.
The Rossi X-ray Timing Explorer (RXTE), launched in December 1995 from Kennedy Space Center, Fla., was designed to observe fast-moving neutron stars, X-ray pulsars and bursts of X-rays that brighten the sky and disappear.
Some pulsars spin over a thousand times a second. A neutron star generates a gravitational pull so powerful that a marshmallow impacting the star's surface would hit with the force of a thousand hydrogen bombs.
Magnetars, the most magnetic stars known, aren't powered by a conventional mechanism such as nuclear fusion or rotation, according to Dr. Vicky Kaspi. "Magnetars represent a new way for a star to shine, which makes this a fascinating field," said Kaspi.
Although not totally understood yet, magnetars have magnetic fields a thousand times stronger than ordinary neutron stars that measure a million billion Gauss, or about a hundred-trillion refrigerator magnets. For comparison, the Sun's magnetic field is only about 5 Gauss.
In the constellation Cassiopeia, approximately 18,000 light years from Earth, a magnetar named 1E 2259 is being studied. It suddenly began bursting in June 2002, with over 80 bursts recorded within a 4-hour window. Since then, Magnetar 1E 2259 hasn't disturbed the depths of space.



Using RXTE, astronomers can study how gravity works near black holes and observe changes in X-ray brightness that last for a thousandth of a second, or for several years. They also can monitor explosive wavelengths not able to be seen in visible light.
Eta Carinae is an extremely massive star in our galaxy, and an extremely unstable one. Since 1996, a science team has been monitoring the X-ray flux from this region using RXTE. As RXTE continues to provide the first detailed monitoring of the X-ray emissions of Eta Car, coordinated observations are helping answer many scientists' questions. HST Image Credit: Jon MorsAstronomers will be able to study this violent and bizarre space phenomenon in greater dimension when NASA's Swift Gamma-Ray Burst Explorer is launched in mid-2004. Swift will be about 20 times more sensitive to magnetar bursts than any other satellite. This research project is a cooperative endeavor between NASA's Marshall Space Flight Center, the National Space Science and Technology Center and several Alabama universities.

A neutron star, located 40,000 light years away, is generating the most intense magnetic field yet observed in the Universe at over 1 x 1014 Gauss! In contrast, the Earth's magnetic field varies from .25 to .65 Gauss, making the field strength of a so-called "magnetar" one thousand trillion times stronger. "The magnetic field generated by this star is truly incredible," Kouveliotou said, "It is so intense that it heats the surface to 18 million degrees Fahrenheit."
This discovery was made with data from RXTE and ASCA (the Japanese-American Advanced Satellite for Cosmology and Astrophysics) combined by Dr. Chryssa Kouveliotou of Marshall Space Flight Center, Dr. Stefan Dieters and Professor Jan van Paradijs of the University of Alabama in Huntsville, and Dr. Tod Strohmayer of Goddard Space Flight Center.In November 1996, the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory detected the magnetar flaring up and emitting gamma-rays. Kouveliotou used time she was allotted on Rossi X-ray Timing Explorer to take a closer look in the X-ray at the behavior that followed. During Nov. 5-18, 1996, RXTE collected several hours worth of data as bursts came in a "bunching" mode that had not been seen before. Following Kouveliotou's observations, RXTE kept watching to provide data for Strohmayer who was allotted time to observe the source during its less active phase. The result was a complementary data set that led to collaboration. "Combining our data gave us both the capability to make a more sensitive search as well as provide a way to verify each other's analysis of the data," said Strohmayer.They also looked through older data taken by ASCA in 1993 when the source was not bursting. Along with the observation of a changing pulse period, the ASCA data was central in establishing that this source was associated with a supernova remnant.

Their finding confirms the existence of the magnetar as a class of neutron stars. Neutron stars themselves, the tightly-packed neutron cores of exploded stars, have very strong magnetic fields but a magnetar has fields 100 times stronger. "The magnetic field generated by this star is truly incredible," Kouveliotou said, "It is so intense that it heats the surface to 18 million degrees Fahrenheit. Periodically, the field drifts through the crust of the neutron star, exerting such colossal forces that it causes a "starquake." The "starquake" energy is then released as an intense burst of low-energy gamma-rays."Objects that emit these bursts of gamma-rays are called Soft Gamma Repeaters. When they burst, they can give off as much energy in a second as the Sun does in an entire year! The magnetar RXTE observed, also known as SGR 1802-20 (SGR being short for Soft Gamma Repeater), was first discovered because it gives off soft gamma-ray bursts.The very first Soft Gamma Repeater was observed in 1979. Their origin has been debated ever since. With this discovery by Kouveliotou's team, researchers have come to believe that the origin of Soft Gamma Repeaters does indeed lie in the 'starquake' phenomena of magnetars. 

The magnetar theory was first 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. How many neutrons stars fall into this special class? 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 with such superstrong magnetic fields 'brake' or 'cool down' over time, in fact SGR 1806-20, which is observed to be spinning once every 7.5 seconds actually slows down 3 milliseconds every year. "We found that the pulsar was slowing down at a rate that suggested a magnetic field strength of about 800 trillion Gauss, a field strength similar to that for so called magnetars predicted by previous theoretical work," said Kouveliotou. Because of the high field strength, it is very difficult to observe stars like this in radio waves or X-rays. This means there could be thousands or millions of these stars scattered throughout our Galaxy, perhaps even accounting for the large number of observed supernova remnants without detectable neutron stars at their centers. Kouveliotou says, "The importance of this discovery goes beyond just adding a new oddity to the list of star types. It ties together two rare, very peculiar classes of stars we have been puzzling over, and puts the evolution of neutron stars and even galaxies in a new light." She added, "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."



(source: NASA)