Mysterious spinning super-magnetic stars:

What the Heck is a Magnetar?

NASA artist depiction of a magnetar in deep space
NASA artist depiction of a magnetar
A magnetar is a weird duck, indeed. It's a spinning neutron star with a super-strong magnetic field a thousand trillion times stronger than Earth's magnetic field. To visualize it, you have to know about neutron stars, pulsars and SGRs. Astronomers have wondered for a long time why certain supernova explosions create magnificent nebulas, yet leave no spinning pulsar at the center. This abnormal situation has been a problem for astronomers trying to calculate star births and deaths and the ages of galaxies and the universe.

An Extraordinary Flare

On December 27, 2004, astronomers detected a flash of energy from across our Milky Way galaxy so powerful that it bounced off the Moon and lit up Earth's upper atmosphere.

The flash, which lasted more than one-tenth of a second, was brighter than anything ever detected from beyond our Solar System.

NASA and European astronomy satellites and many radiotelescopes on Earth recorded the flash.

Scientists discovered the flash of energy came from a magentar ­ the neutron star SGR 1806-20 ­ some 50,000 lightyears away in an area of Earth's sky known as the constellation Sagittarius.

Its apparent magnitude was brighter than a full Moon and all previously recorded star explosions. Most of the energy was in invisible gamma-rays, which are far more energetic than visible light or X-rays.

SGR 1806-20 is known to be a soft gamma repeater (SGR) because it randomly flares up and releases gamma rays. Only four SGRs are known.

The December 2004 giant flare on SGR 1806-20 was millions to billions of times more powerful than typical SGR flares.

For one-tenth of a second, the giant flare unleashed more energy than the Sun emits in 150,000 years.

Magnetic fields surrounding the magnetar probably was responsible for the outbursts.
Understanding began to dawn on scientists in 1979 when gamma ray detectors on nine spacecraft spread out across our Solar System recorded an intense burst of radiation. While the energy spike lasted just 2/10th of a second, it carried as much energy as our Sun releases in 1,000 years. That compared with most such bursts, which release only as much energy as the Sun releases in one year.

The big radiation spike was followed by a 200-second wave of energy pulsing every eight seconds. The radiation burst was coming from a supernova remnant known as N49 in the galaxy known as the Large Magellanic Cloud.

Scientists spotted something odd right away. N49 was only a few thousand years old, yet its eight-second spin rate would have been typical of a much older neutron star. Something was putting the brakes on and slowing down the spinning pulsar.

In 1986, astrophysicists realized they had two more objects like N49. Each sent out low-energy gamma rays in repeated bursts, while gamma ray bursts from other sources usually are one-time events.

They dubbed this new kind of deep-space object soft gamma repeater, or SGR for short. The N49 object was designated SGR 0526-66 for its position in the sky. The others were labeled SGR 1806-20 and 1900+14. Both are in our own Milky Way galaxy. SGR 1806-20 is one of the most active, most energetic of the Soft Gamma Repeaters.

From 1986, the mysterious Soft Gamma Repeaters were recognized as a separate, very peculiar class of star because of their telltale outbursts. But, exactly how were they different?

There were numerous theories until November 1996 when an instrument known as the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory (GRO) spacecraft detected a new energy outburst from SGR 1806-20.

Then the Rossi X-ray Timing Explorer (RXTE) spacecraft captured several hours worth of data as bursts came in bunches that had not been seen before. Combining data from both satellites gave astronomers the ability to make a more sensitive search andto verify analyses of the data.

RXTE carries instruments that read data quickly. While most telescopes really take time exposures, RXTE's instrument known as the Proportional Counter Array acts like a fast electronic counter which searches for a pattern in the X-rays it receives from deep space.

The new information was compared with older data which had been gathered by Japan's Advanced Satellite for Cosmology and Astrophysics (ASCA) spacecraft in 1993. It had observed SGR 1806-20 while it was not sending out burst. ASCA data was important as astronomers established that the SGR was associated with a supernova remnant.

NASA artist concept of magnetar SGR locations along the Milky Way
NASA artist concept of where magnetar
SGRs may be located along the Milky Way
Finding the pulses in the RXTE data allowed astronomers to go back and also find it in the ASCA data, which removed anydoubt that the pulses could be from some previously unknown object in RXTE's field of view.

In the time between the ASCA and RXTE observations, SGR 1806-20 had slowed by 8/1,000th of a second. That difference might seem miniscule, but it happened in less than four years to an object with more mass than our Sun.

Not ordinary pulsars. Having established that SGR 1806-20 is associated with a rapidly slowing pulsar, the astronomers need to figure out what might fit that profile. Of course, proving what something is sometimes involves proving what it is not. The scientists thought that SGRs were objects they referred to as magnetars, but first they had to eliminate objects other than ordinary pulsars as the sources. Then they had to rule out possibilities other than magnetars as the answer.

The first possibility they had to examine was accretion where material from another star is scooped up by the pulsar, or the magnetar. Radio telescope observations at the National Radio Astronomy Observatory ruled out accretion when they showed that SGR 1806-20 coincides with a supernova remnant known as SNR G10.0-0.3. Radio broadcasts from that supernova remnant suggest a compact shape which may be orbiting a nearby massive blue star every 10 years. Since 1806's own stellar wind is too powerful to let material fall inward, it can't be an accreting pulsar.

Astronomers found that the pulsar was slowing down at a rate that suggested a magnetic field strength of about 800 trillion Gauss, which is a strength similar to that predicted in theory for magnetars. By comparison, Earth's magnetic field is a mere 0.6 Gauss at the poles. The maximum created in laboratories is 1 million Gauss. Normal radio pulsars reach about 1 trillion to 5 trillion Gauss, strong but still short of a magnetar.

Extraordinarily hot. The strong magnetic field keeps the star very hot, at about 10 million degrees C (18 million deg. F) at the surface. That powers the X-rays coming from its rotating surface.

Neutron stars are the only stars with a solid surface crust about six-tenths of a mile deep covering a thick fluid of neutrons over either a superfluid or solid core of subatomic particles. On the star's surface, a chunk of magnetizable metal like iron would feel a force equal to 150 million times the Earth's gravitational pull on it. Movement of that strong magnetic field would wrinkle the crust of the neutron star and cause starquakes that would be the source of the soft gamma-ray bursts.

Today, only about 12 magnetars have been found among the millions of regular neutron stars in our Milky Way galaxy and neighboring galaxies.

The crust is believed to be stable in ordinary neutron stars, but in magnetars, the crust probably is stressed by unbearable forces as the colossal magnetic field drifts through it. That deforms the crust and cracks it. Violent seismic waves then shake the star's surface, generating so-called Alfven waves -- reminescent of a Slinky toy -- which energize clouds of particles above the surface of the star. It also drags the star down, slowing it to about a ten-second period in just 10,000 years, which is about the age and speed of SGR 1806-20.

Even stranger. Astronomers think that, eventually, magnetars may become even more strange objects.

Six objects referred to as anomalous X-Ray pulsars (AXPs) are known to be different from most X-ray pulsars. The X-ray colors of those anomalous pulsars are very red compared to something like blue for normal pulsars. Their rotational periods also slow faster than other stars.

Astronomers suggest there could be as many as one to 100 million magnetars have in our Milky Way galaxy. Also, many supernova remnants that lack pulsars actually may 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.

Learn more about magnetars:
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