Text of a press release from the National Radio Astronomy
Observatory
Media contacts: Yvette Estok,
703-306-1070, Dr. Dale Frail,
505-835-7338, Dave Finley,
505-835-7302
Program contact: Robert Dickman,
703-306-1822
Cosmic flasher reveals all
Sept. 25, 1998: 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 magnetar.
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. |
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"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."
Return to Crusty young star
makes its presence felt. |
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