Happy birthday, Magnetars

Scientists note 20th anniversary
of March 5, 1979 gamma-ray burst event

March 5, 1999: Twenty years ago today, a new astrophysical mystery came banging on the door. It was a burst of gamma radiation so strong that it swamped detectors. It was the calling card for a new type of star - the magnetar - that only recently has captured public attention.

Right: An artist's concept depicts the magnetic field lines rising from the surface of a magnetar, and the plasma clouds around the star. Credit: Dr. Robert Mallozzi, University of Alabama in Huntsville.

On March 5, 1979, the gamma ray burst detectors on a number of satellites rang off the scale. Most of the research on the burst was done by a team led by Dr. Thomas Cline of NASA's Goddard Space Flight Center in Greenbelt, Md. But long after the afterglow had faded, the burst continued to reverberate throughout the astrophysics community.

Saturation
"It was a very historic event," said Dr. Gerald Fishman, at NASA's Marshall Space Flight Center. Fishman is principal investigator for the Burst and Transient Source Experiment (BATSE), an instrument aboard the Compton Gamma Ray Observatory used to study gamma ray flashes in space. The March 5, 1979, burst came 12 years before the launch of Compton, and before the other astrophysics satellites now in use. Still, the burst spurred interest in the field.

"Nothing like it has happened since then," Fishman continued. "It was quite exciting because here was a tremendous blast seen by eight or nine spacecraft across the solar system."

The amazing thing is that it saturated every single detector, keeping scientists from getting an accurate measure of the energy peak.

"To this day, there hasn't been another like it," said Dr. Charles Meegan, a co-investigator on the BATSE team, also at NASA/Marshall.

"Our first reaction was that the initial spike was an instrumental effect," said Dr. Chryssa Kouveliotou of the Universities Space Research Association working at NASA/Marshall. Kouveliotou then was a graduate student at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. She was working on her doctorate in astrophysics, using data from the third International Sun-Earth Explorer (ISEE-3) which carried one of the instruments that detected the burst.

Compounding a mystery
Gamma ray bursts had been mystifying astrophysicists since the late 1960s when they were discovered by satellites designed to monitor compliance with a treaty banning nuclear weapons tests in space. The discovery of this phenomenon led scientists to piggyback small detectors on a number of science satellites including interplanetary probes. By looking at the different times that the signals arrived at various satellites, the scientists could get a better fix on the location of the burst sources.

And that's just what they did with the March 5 event. To everyone's amazement, the triangulation pointed to the Large Magellanic Cloud, a satellite galaxy near our own Milky Way galaxy.

"There was a lot of debate about whether it really was from the Large Magellanic Cloud, or was a coincidental overlap," Meegan said. Until then, scientists had thought that most bursts occurred within our galaxy. For the bursts to come from outside the galaxy would require an immense amount of energy in order to appear so strong here.

"A lot of people believed this was an LMC event," Kouveliotou said. "A great many others did not, simply because of the energetics involved."

"It was very strong," Meegan said. "We wondered, Why should the strongest burst be far away? That led us to think that we are dealing with something different."

But the error box was the smallest that had ever been established for a gamma ray burst, and put the apparent source inside the remnant of a supernova known as N49.

Pulses in the afterglow
The mystery deepened when scientists examined the tail which showed a series of pulses that indicated something was rotating and beaming energy.

"That was the astounding thing," Meegan continued. "No one had never seen any period before. That helped convince a lot of people, incorrectly, that gamma-ray bursts are from neutron stars in our galaxy."

A pulsar - a rapidly rotating neutron star - does not really pulse. It actually beams radiation continuously from its north and south magnetic poles. The magnetic poles are normally offset from the geographic poles, so the star's rotation sweeps the beam across the sky like a lighthouse. For an observer who happens to be in the right place, the star appears to pulse on and off.

"The 8-second period seen in the tail was very first coherent modulation observed in a gamma ray burst," Kouveliotou explained.

Making a neutron star - and a magnetar - starts (1) with a massive star that has burned up all of its fuel, then (2) collapses and causes a massive explosion, the supernova. that blows off the outer layers and (3) compresses the core. Soon, all that is left is a shell of expanding gas (not always this pretty or symmetrical) and a rapidly spinning neutron star at "ground zero." If the original star was spinning fast enough and had a strong enough magnetic field, the neutron star is a magnetar.

The March 5, 1979, event was not the first or last gamma-ray eruption from the LMC object. Dr. Evgeny Mazets of the Ioffe Institute in Moscow noticed that a series of 16 events had come from the same general area of the sky, and that the energies of each lay in the region of the electromagnetic spectrum where the nomenclature changes from X-rays to gamma rays.

During 1985-86, Dr. Kevin Hurley of the University of California at Berkeley collected data obtained by a number of scientists and realized that some bursts seemed to be coming from the same general part of the sky, on the plane of the Milky Way galaxy. International collaboration led to discovery of another source at 1806-20 (18 hours, 6 minutes right ascension, -20 deg. declination).

It was time to try to put a name on the new sources. During an international space science meeting in Toulouse, France, in 1986, a group of 50 astrophysicists discussed what had been learned to date.

The 'birth' of magnetars
They also selected a name. Soft Gamma Repeater - SGR - was easier to say than Hard X-ray Repeater - HXR - although both describe the spectral range and fact that they have repeated bursts.

At this point they had just three SGRs: the March 5, 1979 object at 0526-66, Hurley's object at 1806-20, and a third at 1900+14 (again, the numbers indicate the position in the sky). For a long time, SGRs were listed as a peculiar subset of the larger mystery of Gamma Ray Bursts, beasts that appear at totally random times and, as astronomers gradually learned, totally random locations in the sky.

Eventually, BATSE led scientists to separate the two. BATSE showed that the true Gamma-Ray Bursts are scattered across the sky, and not grouped along the plane of the Milky Way galaxy. That means that they are associated with the deep cosmos rather than just with our galaxy. Later discoveries proved that the bursts are deep in the universe and thus almost unbelievably powerful.

Meanwhile, the SGR mystery headed towards resolution. In 1992, Dr. Robert Duncan of the University of Texas in Austin and Dr. Chris Thompson of the University of North Carolina at Chapel Hill formulated their magnetar theory.

Magnetars live a fast and furious youth and then quickly go out to pasture. The current theory is that for about their first 10,000 years they are Soft Gamma-ray Repeaters. Their burst activity drops sharply and for the next 30,000 year they are Anomalous X-ray Pulsars. All the while, the magnetic field is putting the brakes on the magnetar, slowing its rotation and expending energy through starquakes and magnetic field realignments. After 30,000 to 100,000 years, the AXP is just a dark, spinning neutron star - a "dead" magnetar that is virtually undetectable. Because a magnetar's active phase is so brief, the implication is that the galaxy is filled with millions of dead magnetars.

This was a radical new concept. When a massive, rapidly rotating star explodes, it compresses its core to a diameter of about 20 km (12 mi) and having a density so great that a pinhead of neutron star material would weigh as much as a battleship. It's also so hot that for the first 30 seconds or so it circulates as hot neutron liquid rises to the surface, cools, and sinks. This motion generates a magnetic field. If the star is spinning at 200 rotations/second or more (more than 360 times faster than an old 33-1/3 record), it sets up a dynamo effect that generates a magnetic field 1,000 times stronger than that of "ordinary" neutron stars. A magnetar is born.

After the neutron star cools, it forms a 1 km-thick (0.625 mi) crust of iron nuclei jam-packed with almost no space between each other. They have increasingly large atomic numbers, and are increasingly bloated with neutrons, with greater depth. as you go down deeper.

Right: A cross-section diagram shows a neutron star in its first seconds of life. It is still a superhot liquid with two or three layers of convection carrying heat to the surface. If the neutron star is spinning at more than 200 rotations/second, it sets up a dynamo effect that forms an intense magnetic field, and a magnetar is born.

"In ordinary neutron stars the crust is stable, but in magnetars, the crust is stressed by unbearable forces as the colossal magnetic field drifts through it," said Duncan. "This deforms the crust and sometimes cracks it." Violent seismic waves then shake the star's surface, generating Alfvén waves - the electromagnetic equivalent of a Slinky toy - which energize clouds of particles above the surface of the star. These cause most of the bursts attributed to SGRs.

Initially, Duncan and Thompson offered their theory to explain gamma ray bursts and, possibly, SGRs. In time, they and other scientists realized that while the explanation for bursts remains elusive, they probably had found the solution for SGRs and for another odd character, the Anomalous X-ray Pulsars (AXP). For some years, scientists had been looking at AXPs, neutron stars associated with young supernova remnants, but with but with rotation periods much longer than found previously in young neutron stars.

Observational confirmation
Support for the magnetar theory came early in 1998. In 1996, the Rossi X-ray Timing Explorer (RXTE) had observed SGR 1806-20. Kouveliotou and her colleagues discovered a period within persistent X-ray emissions. They then looked at earlier observations by Japan's Advanced Satellite for Cosmology and Astrophysics (ASCA) and found the same period. She found that SGR 1806-20's rotation was slowing at the rate of 1 second every 300 years. What seems like a miniscule effect, Kouveliotou calculated, required an immense cause: an intense magnetic field that is applying the brakes to the neutron star.

Those results were published in Nature on May 21, 1998. Just a month later, a fourth source, SGR 1627-41, was discovered.

SGR outbursts come in two forms. Most are starquakes that occur when the kilometer-thick metallic crust shifts and pumps energy into the plasmasphere around the neutron star. Such an event is depicted here. The really big bursts, like the March 5th 1979 event, are caused by sudden readjustments of the. (Images by Dr. Robert Mallozzi, University of Alabama in Huntsville)

Meanwhile, things were picking up. Kouveliotou, Hurley, and others were observing SGR 1900+14, first in April with ASCA, then on May 26 with BATSE when it erupted, and then with RXTE during June 1-9 where they found the SGR's spindown rate could be measured even in the space of a few days.

Four Soft Gamma Repeaters are currently (2000) known: SGR 0525-66 (discovered 1979) SGR 1806-20 (discovered 1979/1986) SGR 1900+14 (discovered 1979/1986) SGR 1627-41 (discovered 1998) The numbers give the position in the sky - SGR 0525-66 for example has a right ascension of 5h25m and a declination of -66°. The 1979/1986 years are given that way because these have been first seen in 1979, but soft gamma repeaters were only recognized as a separate class of objects (rather than 'normal' gamma ray bursts) in 1986.

Then, on August 27, 1998, SGR 1900+14 really sounded off. Hurley saw a clear series of pulsations in data recorded by a detector on the Ulysses probe out near Jupiter. The Advanced Satellite for Cosmology and Astrophysics and the Rossi X-ray Timing Explorer also recorded the burst.

It had almost the same apparent brightness as the March 1979 event burst by SGR 0526-66. But because the Large Magellanic Cloud containing SGR 0526-66 is about 8 times more distant than SGR 1900+14, SGR 0526-66 was intrinsically 64 times brighter.

As with the March 5, 1979 burst, a long tail with strong pulsations was recorded. The intensity and other factors led scientists to confirm that SGR 1900+14 is a magnetar, like SGR 1806-20. The burst apparently was caused by an out-of-control magnetic field adjusting itself in a manner similar to what happens inside solar flares.

At the head of the class
After almost two decades, SGRs had graduated to be the most vocal of a new class of stars, magnetars, also comprising AXPs and "dead" magnetars that have wound down and become silent and, to Earthlings, invisible.