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.
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Birthday album Many of the images in this story and earlier magnetar stories from Science@NASA are available on an image page as high-resolution 1280x1024-pixel JPGs sized for photography on an Imagecorder or other computer-to-film display. The page has 19 images with thumbnails about 10K to 37K in size, so the page will take a couple of minutes to load. Other images are available on the magnetar images page from an earlier story. |
"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 almost boils 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.
Full size image available through contact |
"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 periods indicating they might be much older.
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.
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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. 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 50 times more distant than SGR 1900+14, SGR 0526-66 was at least 2,500 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 realigning 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.
So what's next?
"We keep observing SGRs and AXPs with every satellite we've got," Kouveliotou said. "You never know what more surprises are out there."
That means whenever possible, watching with ASCA, Rossi X-ray Timing Explorer, and - later this year - the Chandra X-ray Observatory.
Web Links
Crusty young star makes its presence felt: Gamma ray flash zaps satellites,
illuminates Earth, and sheds light on several mysterious stellar events. (Sept.
28, 1998)
A whole lot of shakin' going on: Starquakes lead to discovery of first new Soft Gamma Repeater in 19 years (July 9, 1998)
Magnetar discovery announcement including more details, interviews and more illustrations (May 20, 1998)
Looking for Pulsars in the Fast Lane . Scientists are looking for bizarre, short-lived, powerhouse stars that burst with some of the brightest energy in the universe. Using the Chandra X-Ray Observatory, they hope to find some of the few that may exist. (Sept. 21, 1998)
Magnetars.com website