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A concept image of what a magnetar might look like

Magnetar Animation (QT 933kb)

Magnetar and Gamma-ray Burst Facts

• the U.S. Dept. of Defense launched the "Vela" satellites in the late 1960s to verify a nuclear test-ban treaty by monitoring gamma radiation from atomic explosions. Brief bursts of gamma rays were found.
• they came randomly from all directions in the sky and were not man-made.
• a new natural phenomenon, "gamma-ray bursts" (GRBs), was discovered and announced to astronomers in 1973.
• It wasn't until 1987 that SGRs were clearly recognized as a distinct set of objects.
• it is now believed that most of these bursts come from very far away, near the edge of the observable universe.
• they are called "cosmological gamma-ray bursts" and scientists still do not understand what causes them.
• even at a distance of about 200,000 km (160,000 mi), a magnetar's field would be as strong as a refrigerator magnet is up close.
• the field could certainly erase your credit cards and suck pens out of your pocket as far away as half the distance to the Moon, perhaps farther.
• a neutron star's gravity is so intense at the surface that Earth's tallest mountain ranges would be flattened to about the height of an ant.^1ah
• because a magnetar's field is very strong, it spins down via magnetic waves very quickly
• field strengths in excess of 10¹⁴ Gauss are required for the formation of a Magnetar.

Magnetars, in the simplest sense, are neutron stars with a super-strong magnetic field that is a quadrillion times stronger than Earth's. Neutron stars are left when a massive star expends itself in a type-II supernova. Most become pulsars when rotation of the neutron star's magnetic field produces a repeating, clocklike signal in radio, light, even X-rays (right) and gamma rays. In the case of the Soft Gamma Repeaters (SGRs), the answer also appears to be a magnetar. It has been postulated that this incredibly powerful magnetic field actually slows the remnants' rotation.

This same magnetic field strength is thought to cause a shifting in the semi-liquid crust of the parent object, which is called a starquake. These uphevals can pump enough energy into the surrounding gases to generate bursts of soft gamma radiation. These outbursts led to the identification and discovery of the first SGR in 1979 by a Soviet treaty-monitoring sattelite. For almost two decades, scientists speculated about the source - eventually proposing a new class of highly magnetized stars called magnetars.

This discovery goes beyond adding a star type, but ties together two rare, very peculiar classes of stars which have been puzzling scientists everywhere, and redefines the evolution of neutron stars and perhaps even galaxies. It may also increase the population of supermassive objects within our galaxy to include a few hundred million undiscovered magnetars.

The discoverers of this object are Cryssa Kouveliotou (Universities Space Research Association (USRA) at NASA's Marshall Space Flight Center), Dr. Jan van Paradijs (University of Amsterdam and the University of Alabama in Huntsville), Dr. Stefan Dieters ( University of Alabama in Huntsville), and Dr. Tod Strohmayer (NASA's Goddard Space Flight Center, Greenbelt, MD).

Their findings strongly support the magnetar theory offered 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. >> Click here for the full story of their discovery. << Their theoretical model has the neutron star going through a violent afterlife lasting about 10,000 years. While many colleagues discounted the concept, saying that internal pressures and other factors would keep a star from generating such an intense field, Duncan said that "We were just trying to understand the origin of the magnetic fields of radio pulsars, which are the ordinary, familiar type of neutron stars."^1ai

One of the mysteries surrounding Magnetars is why a large number of supernovas create magnificent nebulas yet leave no pulsar at the center. This anomaly poses a problem for theorists trying to calculate the rates of star birth and death - and eventually, the ages of galaxies and the universe. The pulsar problem seemed rather unique because the known pulsar magnetic fields are relatively very weak compared to what is possible in forming neutron stars. As a result, SGRs were not recognized as a separate class until 1986, although all three had been discovered in 1979.

The "Index" Event and subsequent SGR occurences

The Index event, called the Sagittarius burster, occurred on January 7, 1979, and during the first 3 months of 1979, three of the four known SGRs were discovered. Although the Index event was notable in a historical sense, the real show was the GRB 790305 event.

On March 5, 1979, gamma ray detectors on nine spacecraft recorded an intense radiation spike of energy in the Gamma-ray frequencies. Lasting just 2/10th of a second, it released as much energy as the sun releases in 1,000 years. It was then followed by a 200-second emission that showed a clear 8-second pulsation period.

The Right Ascension (RA) and Declination (DEC) tied the burst to a supernova remnant known as N49 in the Large Magellanic Cloud. Immediately, scientists recognized something odd. N49's youth - it's only a few thousand years old - contrasted with its 8-second spin, typical of a much older neutron star. The burning question of the moment was "what is slowing the rotation (hence pulsation) of this supermassive object ?"

The mystery expanded in 1986 when astrophysicists realized that they had two more objects like this. Each emitted repeated low-energy gamma rays, and were thus dubbed Soft Gamma Repeaters, or SGRs. The object associated with N49 was designated SGR 0526-66. The other two SGR candidates were SGR 1806-20 at 14 kiloparsecs (one of the most active), and SGR 1900+14, both in the Milky Way.

In November 1996, the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory detected SGR 1806-20 flaring up again. The X-ray activities that followed were closely examined. RXTE also captured several hours worth of data as bursts came in a "bunching" mode that had not been seen before. RXTE kept watching SGR 1806-20 to provide data of SGR 1806-20 in its quiescent phase. The result was a complementary data set that led to collaboration. This provided a more sensitive search as well as a way to verify analysis of the data."

As the name implies, the Rapid response X-ray Transient Experiment (RXTE) carries instruments that read data quickly. Where most telescopes take time exposures, the Proportional Counter Array aboard the RXTE acts like a fast electronic counter which, combined with its size, was highly effective in searching for a pattern in the X-rays. A candidate periodic signal at 7.5 seconds was found, but confirmation that the signal was also in the other datasets was needed. The study was complicated by the need to carefully remove the data segments which had SGR bursts in them - in looking for such a weak pulsar signal, the bursts could totally mask the weaker modulations.of the SGR. However, knowing what period you are looking for gives the astrophysicist a great advantage in sensitivity.

The pulsed signal in the RXTE data also was visible in the ASCA data, which removed the last shred of doubt that the pulsed signal could possibly be from a previously unknown object within the RXTE field of view. Herein lies the first mystery - between the ASCA and RXTE observations, SGR 1806-20 had slowed by 8/1,000th of a second. The difference would be negligable to you or I, but this happened in less than four years to an object with more mass than our sun.

More investigation was done by a colleague of Dr. Kouveliotou, Dr. Jeff Kommers of the Massachusetts Institute of Technology. Using a different approach, he came up with 7.5 mseconds (7.5/1000 of a second). Having established that SGR 1806-20 is associated with a pulsar and that it is rapidly slowing, Scientists suspected that SGRs were, in reality, magnetars. However, to confirm this, they first had to eliminate objects other than pulsars as the sources, and then eliminate possibilities other than magnetars as the answer.

The first possibility was a simple accretion model where material from another star is absorbed by the pulsar - or the magnetar. Radio telescope observations by Dale Frail at the National Radio Astronomy Observatory helped rule out the accretion model. In his research, he showed that SGR 1806-20 coincides with a supernova remnant, SNR G10.0-0.3, whose radio broadcasts suggest a compact shape. It is also may be orbiting a nearby massive blue star every 10 years. SGR 1806-20's own stellar wind is too powerful to let material fall inward, so it can't be an accreting pulsar.

An ordinary pulsar emits radio waves that rotate with the remnant. When the object is oriented so that these beams sweep across the Earth, radio telescopes can detect these pulses. This pulsar was found to be 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. The Earth's magnetic field is a mere 0.6 Gauss at the poles, and the best sustainable field in laboratories is 1 million Gauss - in a small volume. Normal radio pulsars reach about 1 trillion to 5 trillion Gauss. Within the field of a Magnetar, magnetism itself can keep the star hot - about 10 million degrees C (18 million deg. F) at the surface, and are thought to power the X-rays coming from its rotating surface.

At the surface of the remnant a chunk of magnetizable metal like iron would feel a force equal to 150 million times the Earth's gravitational pull on it. At this intensity, the magnetic field's movements wrinkle the crust of the neutron star and cause starquakes that are believed to be the source of the soft gamma-ray bursts. 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.

In neutron stars the crust is stable, but in magnetars the crust is stressed by tectonic forces as the magnetic field becomes more dynamic. This deforms the crust and sometimes cracks it. Violent seismic waves then shake the star's surface, generating Alfven waves, which energize clouds of particles above the surface of the star. This process also increases the drag on the star, slowing it to about a 10-second period in just 10,000 years - about the age and speed of SGR 1806-20.

AXPs

There are six known Anomalous X-ray Pulsars (AXPs) that are different from the bulk of the X-ray pulsars. In terms of colors, the X-ray colors of the anomalous pulsars are very red compared to the "normal" blue pulsars. AXP rotational periods also slow faster than other stars, and their pulse periods are close together. All of them had a pulse period of 6 to 10 seconds, which is very different from what you find with normal X-ray pulsars, which have pulse periods as short as less than a tenth of a second and as long as half an hour. Since they slow down rapidly, there are only a few visible at any one point in time.

This leads astrophysicists to believe that even though there may be many of them, most of them are now dormant or dead. If the 10 or so SGRs and AXPs are magnetars, each less than 10,000 years old, then they probably form about once every thousand years. Some scientists think that at least 1 million magnetars have formed in our galaxy, and perhaps as many as 30 to 100 million.^1ai This has lead others to think that many of the supernova remnants that lack pulsars actually have them in the form of invisible, dead pulsars that exploded as supernovas, sputtered as SGRs concealing magnetars, then faded through the AXP stage to become invisible. Others may be made visible with more sensitive instruments like NASA/Marshall's Advanced X-ray Astrophysics Facility (Chandra), now in orbit, and credited with many new discoveries already.

Studying Magnetars

Astronomers have found evidence for the most powerful magnetic field ever seen in the universe. They found it by observing a short-lived "afterglow" of subatomic particles ejected from a magnetar.^77a The afterglow is believed to be the aftermath of a massive starquake on the neutron star's surface. Dr. David 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 is thought to come from particles ejected at nearly the speed of light from the surface of the neutron star interacting with an extremely powerful magnetic field. This is the first time this predicted phenomenon has been seen so clearly from a suspected magnetar. Astronomers have seen one type of their predicted activity previously, and now a completely different piece of evidence confirms that this object is a magnetar.

On August 27, the SGR called 1900+14 underwent a tremendous burst, the likes of which had not been seen since 1979. 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. Particle "winds," predicted by theory to carry as much energy as the flashes of hard X-ray emission and are important in slowing down the spinning magnetar, were discovered, which allows astronomers to pinpoint the exact location of the SGR to allow further study of the magnetar with other powerful telescopes. In time, the free-flowing particle wind will inflate a type of nebula called a plerion.

Magnetars might naturally acquire a large recoil velocities at birth, via the "neutrino rocket effect," which is still being studied and debated. It is possible that this could explain the large observed displacements of SGRs from the centers of their associated SNRs, but the theory has been widely met with skepticism.

In 1995 Duncan, et al. published a paper in the Monthly Notices of the Royal Astronomical Society with more details (Thompson & Duncan, M.N.R.A.S. 275, 255). In it, they outlined seven different ways to estimate the magnetic field of the March 5th burster, all of which seemed to indicate a field greater than 10¹⁴ Gauss.

In particular, they argued that if, and only if, the field exceeds 10¹⁴ Gauss, it could:

1. Spin down the star to an 8.0 s period in the age of the SNR (as mentioned above).
2. Provide enough energy for the March 5th event, a putative magnetic flare.
3. Account for the short, 0.2-second duration of the March 5th event's hard spike. This is the time needed to make a large-scale magnetic readjustment, since magnetic disturbances must travel through the star.
4. Drive magnetic dissipation QUICKLY enough to explain SGR activity in a time of order 10,000 years, the ages of SGRs, as inferred from their associated SNRs.
5. Provide enough energy to heat an SGR's interior and cause the steady X-ray glow of SGRs. ("the X-ray point sources").
6. Render the hot gas of particles which emitted the March 5th event's soft tail (and normal SGR bursts) nearly transparent to X-rays. This is necessary to explain why SGR bursts are so extraordinarily bright. This important point was first noted by Bohdan Paczynski of Princeton University in 1992, shortly after our first magnetar paper.
7. Hold down, with magnetic forces, the hot gas of particles which emitted the March 5th event's soft tail.

Conclusions:

• For the remnant to spin down to such a slow rotation rate in the age of the associated SNR requires a very strong magnetic field.
• Kouveliotou et al. have, for the first time, measured the present RATE of spindown for an SGR.
  • This allows a direct estimate of the magnetic field, via the same method used in radio pulsar studies.
  • the magnetic field of SGR 1806-20 is now inferred with the same confidence as the fields of radio pulsars.
  • The data of Kouveliotou et al. also support the idea that SGRs are essentially the same kind of star as the "Anomalous X-ray Pulsars" (AXPs).
    • Six of these mysterious AXPs are now known in our galaxy, three of which were discovered in just the last 18 months.
    • AXPs emit X-rays that pulsate with periods of about 10 seconds, are found in young SNRs (at least most of them)
    • They are gradually spinning down.
    • In short, AXPs look very much like SGR 1806-20, except that they have not (yet) been observed to burst.

It is interesting to speculate that BOTH SGRs and AXPs are magnetars.

• If so, a total of 10 young magnetars are now known.
• Since they all lie in SNRs with ages < 10,000 years (in which they presumably formed) one magnetar must form every 1,000 years, on average.
• This is about 10% of the rate of supernovae in the Galaxy.
• If most supernovae form neutron stars, then one could say that roughly 10% of all neutron stars are magnetars, as suggested by Kouveliotou et al.
• However, this might be an underestimate, since 4 of the 10 magnetar candidates were found during the last 18 months (including SGR 1815-13)
• some stars that are neither identified as SGRs nor AXPs might be magnetars.
• more than twenty neutron stars in young SNRs are now known, about 1/2 of which are poorly understood and MIGHT be some manifestation of magnetars.
• we don't yet know how common magnetars are.
• If both SGRs and AXPs are magnetars, then magnetars plausibly constitute more than 10% of all neutron stars, and the fraction might be as large as 50%.
• Because neutron stars have been forming throughout the history of our galaxy, this means that there are almost certainly more than a million old magnetars in our galaxy, and perhaps as many as thirty to one hundred million.
• about 10,000 years after their birth, the magnetic energy sources in these stars begins to run down.
• this agrees with our calculations and observations: no old SGRs or AXPs are found.
• the vast majority of magnetars have long ago ceased bursting and ceased emitting bright X-rays.
• they are now slowly-rotating, dark stars, quietly drifting through the Galaxy, and difficult to detect.¹⁷

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