Sept. 29, 1998
The wave hit the night side of the Earth and ionized (or knocked electrons out of) the atoms in the upper atmosphere to a level usually seen only during daytime. This astonishing blast of ionization was detected by Prof. Umran Inan of Stanford University. "It is extremely rare for an event occurring outside the solar system to have any measurable effect on the Earth," Inan said. It was so powerful that it blasted sensitive detectors to maximum or off-scale on at least seven scientific spacecraft in Earth orbit and around the solar system.
The wave of radiation emanated from a newly discovered type of star called a magnetar. Magnetars are dense balls of super- heavy matter, no larger than a city but weighing more than the Sun. They have the greatest magnetic field known in the Universe, so intense that it powers a steady glow of X-rays from the star's surface, often punctuated by brief, intense gamma-ray flashes, and occasionally by cataclysmic flares like the one observed on August 27. Astronomers think that all these effects are caused by an out-of-control magnetic field -- a field capable of heating, mixing, and sometimes cracking the star's rigid surface to bits.
In June a team of scientists led by Dr. Chryssa Kouveliotou of NASA's Marshall Space Flight Center in Huntsville, AL, used NASA's Compton Gamma Ray Observatory to detect a series of about 50 flashes from the star, a type called a Soft Gamma Repeater (SGR), known as "SGR1900+14" in the constellation Aquila. During the flashing episode, Kouveliotou's team, in collaboration with Dr. Tod Strohmayer and his colleagues at NASA's Goddard Space Flight Center, Greenbelt, MD, pointed sensitive X-ray detectors aboard NASA's Rossi X-ray Timing Explorer satellite toward the star. They found faint X-rays coming from the star, which pulsed regularly in intensity every 5.16 seconds.
These 5.16-second pulses already had been detected in April, when Dr. Kevin Hurley, University of California, Berkeley, aimed the Japanese/NASA Advanced Satellite for Cosmology and Astrophysics (ASCA) at the star. Comparisons of the ASCA and RXTE data showed that the X-ray pulses were gradually slowing down.
The finding implies that the Soft Gamma Repeater has a magnetic field about 800 trillion times stronger than Earth's magnetic field, and about 100 times stronger than any found anywhere in the Universe. Kouveliotou and her team had earlier found that another SGR was also a magnetar. This was exactly what Dr. Robert Duncan, University of Texas, Austin, and Dr. Christopher Thompson, University of North Carolina, Chapel Hill, predicted in 1992 when they originated the "magnetar" theory.
Before the NASA team could announce these conclusions, SGR1900+14 emitted the tremendous flare of August 27, which was observed by almost every spacecraft with a high-energy radiation detector in space.
"Magnetars seem to answer several mysteries about the structure and evolution of stars," said Kouveliotou. "We think magnetars spend their first 10,000 years as Soft Gamma Repeaters. As they weaken with age and slow their rotation, they become Anomalous X-ray Pulsars -- stars that do not have enough 'juice' to flash anymore, but which emit a steady flow of X-rays for perhaps another 30,000 years. After that, they fade to black and drift for eternity through the heavens. The absence of observable pulsars in some supernova remnants just means that the pulsar's lights have gone out sooner than we expected."
A magnetar forms from the explosion, or supernova, of a very large, ordinary star. The star's heavy center collapses under its own gravity into a dense ball of super-compressed matter 12 miles across. This "neutron star" consists mostly of neutrons in a dense fluid, but the outer layers solidify into a rigid crust of atoms about 1 mile deep, with a surface of iron.
Even with this solid crust, a magnetar is incredibly unstable. Almost unimaginable magnetic fields, about 800 trillion times that of Earth's, cause the crust to crack and ripple in powerful starquakes. The energy released in these explosive starquakes streams out into space as intense flashes of gamma- rays. In the August 27 flare, pure magnetic energy was also released, as the star's entire crust was broken to bits.
"A magnet this strong could erase the magnetic strip on the credit cards in your wallet or pull the keys out of your pocket from a distance halfway to the Moon," said Duncan.
Crusty young star makes its presence felt - Gamma ray flash zaps satellites, illuminates Earth, and sheds light on several mysterious stellar events.
National Radio Astronomy Observatory
Socorro, New Mexico 87801
25 September 1998
"Nature has created a unique laboratory where there are magnetic fields far stronger than anything that can be created here on Earth. As a result, the study of these objects enables us to study the effects of extraordinarily intense magnetic fields on matter," explains Dr. Morris L. Aizenman, Executive Officer in the Division of Astronomy at the National Science Foundation.
Frail, an astronomer at the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, along with Shri Kulkarni and Josh Bloom, astronomers at Caltech, discovered radio emission coming from a strange object 15,000 light-years away in our own Milky Way Galaxy. The radio emission was seen after the object experienced an outburst of gamma-rays and X-rays in late August.
"This emission comes from particles ejected at nearly the speed of light from the surface of the neutron star interacting with the extremely powerful magnetic field," said Kulkarni. This is the first time this phenomenon, predicted by theorists, has been seen so clearly from a suspected magnetar.
"Magnetars are expected to behave in certain ways. Astronomers have seen one type of their predicted activity previously, and now we've seen a completely different piece of evidence that says this is, in fact, a magnetar. That's exciting." Kulkarni said. The new discovery, the scientists say, will allow them to decipher further details about magnetars and their outbursts.
Magnetars were proposed in 1992 as a theoretical explanation for objects that repeatedly emit bursts of gamma-rays. These objects, called "soft gamma-ray repeaters," or SGRs, were identified in 1986. There still are only four of these known. They are believed to be rotating, superdense neutron stars, like pulsars, but with much stronger magnetic fields.
Neutron stars are the remains of massive stars that explode as a supernova at the end of their normal lifetime. They are so dense that a thimbleful of neutron-star material would weigh 100 million tons. An ordinary pulsar emits "lighthouse beams" of radio waves that rotate with the star. When the star is oriented so that these beams sweep across the Earth, radio telescopes detect regularly-timed pulses.
A magnetar is a neutron star with an extremely strong magnetic field, strong enough to rip atoms apart. In the units used by physicists, the strength of a magnetar's magnetic field is about a million billion Gauss; a refrigerator magnet has a field of about 100 Gauss.
This superstrong magnetic field produces effects that distinguish magnetars from other neutron stars. First, the magnetic field is thought to act as a brake, slowing the star's rotation. The earlier discovery of pulsations several seconds apart in three SGRs indicated rotation rates slowed just as predicted by magnetar theory.
Next, the magnetic field is predicted to cause "starquakes" in which the solid crust of the neutron star is cracked, releasing energy. That energy is released in two forms -- a burst of gamma-rays and X-rays and an ejection of subatomic particles at nearly the speed of light. The gamma-ray and X-ray burst lasts no more than a few minutes, while the ejected particles, interacting with the star's magnetic field, can produce detectable amounts of radio emission for several days.
On August 27, the SGR called 1900+14 underwent a tremendous burst, the likes of which had not been seen since 1979. "For a number of years now, I've been routinely looking toward the region of sky where we thought this thing might be," said Frail, "hoping the magnetar would show itself." It did not disappoint; on September 3, the VLA found a new source of radio emission where one had not previously existed. The source quickly faded from view one week later.
The immediate importance of this finding is that it provides a new and independent confirmation of the magnetar model. These impulsive particle "winds," predicted by theory, carry as much energy as the flashes of hard X-ray emission and are important in slowing down the spinning magnetar.
This discovery also allows astronomers to pinpoint the exact location of the SGR to allow further study of the magnetar with other powerful telescopes. "Trying to find this source of gamma-rays was like nighttime sailing with a broken lighthouse; now, we're no longer in the dark, and can study the magnetar for years to come," said Bloom. In time, the free-flowing particle wind can inflate a nebula called a plerion. "This 'windbag nebula' can tell us a lot about the outflow of particles and the burst history of the object," Frail said. "In fact, studying this phenomenon can give us information about the magnetar that we can't learn any other way."
The VLA is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
For more information on magnetars and soft gamma-ray repeaters, see the Background Information which includes a "movie" of the flashing magnetar nebula, as seen by the VLA.
VLA Images of SGR 1900+14, with its short-lived radio emission turned off, left, and on, right. The radio emission comes from the interaction of subatomic particles with the magnetar's powerful magnetic field. The circles indicate the area from within which the X-ray emission of SGR 1900+14 comes.
NASA Headquarters, Washington, DC
Marshall Space Flight Center, Huntsville, AL
May 20, 1998
The discovery confirms the existence of a special class of neutron stars dubbed "magnetars." Magnetars have a magnetic field estimated to be one thousand trillion times the strength of Earth's magnetic field. 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.
The discovery, to be published in the May 21 issue of the journal Nature, was made by a team of astronomers at the Marshall Space Flight Center led by Dr. Chryssa Kouveliotou of the Universities Space Research Association, working with 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.
"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.
Kouveliotou and her team determined the strength of the magnetic field by combining data gathered by NASA's Rossi X-Ray Timing Explorer satellite with data from the Advanced Satellite for Cosmology and Astrophysics, a collaborative mission between Japan and the United States.
"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."
Since these bursts happen quite often and the bulk of their energy is in low-energy (soft) gamma rays, the objects associated with them had been named Soft Gamma Repeaters. 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. The magnetar in question, called SGR 1806-20 by astronomers, was first 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 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.
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.
The magnetar SGR 1806-20 is observed to be spinning once every 7.5 seconds and is slowing down roughly three milliseconds per year. Superstrong magnetic fields cause a neutron star to 'brake' and 'cool down,' making it practically impossible to observe them in radio waves or X-rays. This means there could be thousands or even millions of these dark relics scattered throughout our Milky Way galaxy. This could account for the large number of observed supernovae remnants without detectable neutron stars at their centers.
For more information on magnetars and this discovery, visit NASA Marshall's Space Sciences Laboratory.