Happy birthday, Magnetars
Scientists note 20th anniversary
of March 5, 1979
gamma-ray burst event
SGR 0525-66 in N49 (LMC)
GRB 790305 - (s.a.
P = 8 s;
P' = ... s/s
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.
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
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.
"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.
The burst was
just 2/10ths of a second long - with as much energy as the sun releases in
1,000 years, and was followed by a 100-second tail.
"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
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.
|Out in the
Clouds (left) are two mini-galaxies orbiting our own Milky Way galaxy.
Because they lie in Earth's southern skies, they were unknown to European
astronomers until recorded by Ferdinand Magellan's flotilla that circled
the globe in 1520-21. The Large Magellanic Cloud - where SGR 0526-66 is
believed to reside - is 163,000 to 196,000 light years away; the Small
Magellanic Cloud is ~ 60 kpc. They are trailed
by stars and gas - the Magellanic Stream - apparently stripped out by
tidal forces with our galaxy. At right is an X-ray image of supernova
remnant N49, taken by Germany's Roentgensatellit, the probable host of SGR
0526-66 ("hot spot" in small box at top). Credits: Dallas Parr (CSIRO); W.
Keel (U. Alabama in Tuscaloosa), Cerro Tololo, Chile;
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
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
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
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
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
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
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.
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.
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.
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
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:
The numbers give the position in the sky - SGR 0525-66 for example has a right ascension of
5h25m and a declination of -66°.
- SGR 0525-66 (discovered 1979)
- SGR 1806-20 (discovered 1979/1986)
- SGR 1900+14 (discovered 1979/1986)
- SGR 1627-41 (discovered 1998)
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
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
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.
Magnetar: AXP und soft GRB - Repeater
( Eintrag vom 11.08.2004) ||