NSF PR 98-58 - September 25, 1998
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Cosmic Flasher Reveals All
Astronomers have found evidence for the most powerful
magnetic field ever seen in the universe. They found
it by observing a long-sought, short-lived "afterglow"
of subatomic particles ejected from a magnetar --
a neutron star with a magnetic field billions of times
stronger than any on Earth and 100 times stronger
than any other previously known in the Universe. The
afterglow is believed to be the aftermath of a massive
starquake on the neutron star's surface.
"Where there's smoke, there's fire, and we've seen
the 'smoke' that tells us there's a magnetar out there,"
says Dale Frail, who used the National Science Foundation's
(NSF) Very Large Array (VLA) radio telescope to make
the discovery.
"Nature has created a unique laboratory where there
are magnetic fields far stronger than anything that
can be created here on Earth. As a result, the study
of these objects enables us to study the effects of
extraordinarily intense magnetic fields on matter,"
explains Dr. Morris L. Aizenmann, executive officer
in the Division of Astronomy.
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 emission comes from particles ejected at nearly
the speed of light from the surface of the neutron
star interacting with the extremely powerful magnetic
field," said Kulkarni. This is the first time this
phenomenon, predicted by theorists, has been seen
so clearly from a suspected magnetar.
"Magnetars are expected to behave in certain ways.
Astronomers have seen one type of their predicted
activity previously, and now we've seen a completely
different piece of evidence that says this is, in
fact, a magnetar. That's exciting." Kulkarni said.
The new discovery, the scientists say, will allow
them to decipher further details about magnetars and
their outbursts.
Magnetars were proposed in 1992 as a theoretical explanation
for objects that repeatedly emit bursts of gamma-rays.
These objects, called "soft gamma-ray repeaters,"
or SGRs, were identified in 1986. There still are
only four of these known. They are believed to be
rotating, superdense neutron stars, like pulsars,
but with much stronger magnetic fields.
Neutron stars are the remains of massive stars that
explode as a supernova at the end of their normal
lifetime. They are so dense that a thimbleful of neutron-star
material would weigh 100 million tons. An ordinary
pulsar emits "lighthouse beams" of radio waves that
rotate with the star. When the star is oriented so
that these beams sweep across the Earth, radio telescopes
detect regularly-timed pulses.
A magnetar is a neutron star with an extremely strong
magnetic field, strong enough to rip atoms apart.
In the units used by physicists, the strength of a
magnetar's magnetic field is about a million trillion
Gauss; a refrigerator magnet has a field of about
100 Gauss.
This superstrong magnetic field produces effects that
distinguish magnetars from other neutron stars. First,
the magnetic field is thought to act as a brake, slowing
the star's rotation. The earlier discovery of pulsations
several seconds apart in three SGRs indicated rotation
rates slowed just as predicted by magnetar theory.
Next, the magnetic field is predicted to cause "starquakes"
in which the solid crust of the neutron star is cracked,
releasing energy. 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.
On August 27, the SGR called 1900+14 underwent a tremendous
burst, the likes of which had not been seen since
1979. "For a number of years now, I've been routinely
looking toward the region of sky where we thought
this thing might be," said Frail, "hoping the magnetar
would show itself." It did not disappoint; 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. These impulsive particle "winds,"
predicted by theory, carry as much energy as the flashes
of hard X-ray emission and are important in slowing
down the spinning magneta.
This discovery also allows astronomers to pinpoint
the exact location of the SGR to allow further study
of the magnetar with other powerful telescopes.
"Trying to find this source of gamma-rays was like
nighttime sailing with a broken lighthouse; now, we're
no longer in the dark, and can study the magnetar
for years to come," said Bloom. In time, the free-flowing
particle wind can inflate a nebula called a plerion.
"This 'windbag nebula' can tell us a lot about the
outflow of particles and the burst history of the
object," Frail said. "In fact, studying this phenomenon
can give us information about the magnetar that we
can't learn any other way."
The VLA is an instrument of NSF's National Radio Astronomy
Observatory operated under cooperative agreement by
Associated Universities, Inc.
Editors: For images and more information on
magnetars and soft gamma-ray repeaters see: http://www.nrao.edu/pr/magnetar.background.html
Note to Broadcasters: Images will be available
on Betacam SP. For tape contact Dena Headlee
at 1-888-937-5249.
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