Odd
flashing pulses coming from a super-magnetic star called a magnetar
have astronomers glued to telescopes across the globe.
Magnetars
are particularly energetic versions of neutron stars, which are the burned out
remnants of regular stars.
In March,
astronomers detected a magnetar approximately 10,000 light-years from Earth in
the direction of the constellation Sagittarius, emitting regularly timed radio
pulses. Theory had predicted that due to their high magnetic
fields--100 to 1,000 times stronger than typical radio pulsars--magnetars
would be unlikely to send out radio waves.
The recent
sighting, which relied on the Parkes radio telescope in Australia, is causing
them to rethink fundamental theories about these extreme stars.
"Previous
to our detection there were some theories that explained why you could not get
radio emission from magnetars; obviously those are now incorrect," said
Fernando Camilo of the Columbia Astrophysics Laboratory at Columbia University
in New York.
The
findings were detailed in the Aug. 24 issue of the scientific journal Nature.
Referred to
as XTE J1810-197, the magnetar was first spotted by NASA's Rossi X-ray Timing
Explorer in 2003 when the object abruptly 'came to life' with a strong burst of
X-rays. Then, in 2004, astronomers using the National Science Foundation's Very
Large Array radio telescope found the object was emitting radio waves.
To explain
the anomaly, the scientists presumed the radio waves were emitted from a cloud
of particles flung from the neutron star at the time of the X-ray burst. This
theory was soon proved wrong when Camilo and his colleagues discovered that XTE
J1810-197 was emitting strong radio pulsations
every 5.5 seconds, which corresponds to the estimated rotation rate of this magnetar.
The
research team suspects that the magnetar's mega magnetic field is twisting,
causing the electric currents flowing along its magnetic field lines to change
locations. These currents, they think, are fueling the detected radio pulses.
Beacons
of light
Like their
galactic cousins radio pulsars, magnetars are a type of spinning neutron star
thought to result from the explosive death, or supernova, of a massive star.
These spinning neutron stars emit a constant stream of electromagnetic
particles from their magnetic poles. As the star whips around on its axis, the
particles traveling at near-light-speed sweep out into space. When they shine
toward Earth, astronomers pick up the ejections as pulses with radio and X-ray
telescopes.
A magnetar's
powerful magnetic field means that as the field decays, the star emits
high-energy radiation in the form of X-rays. "The magnetic field from a magnetar
would make an aircraft carrier spin around and point north quicker than a
compass needle on Earth," said David Helfand of Columbia University.
More
dazzling finds
With
further probing, the researchers are finding even more bizarre
traits of this celestial lighthouse.
"Perhaps even
more surprising was the characteristic of this emission, which differs in many
significant respects from 'normal' pulsar emission," said Camilo. For instance,
the brightness of the radio pulses varies from day to day, a phenomenon not
found in the 1,700 or so known radio pulsars.
Another
surprise: Most pulsars get weaker at higher radio frequencies.
"For me,
one of the most spectacular characteristics of the emission is that its
spectrum is apparently flat," Camilo said. That means its brightness is the
same at all frequencies observed, all the way from 350 megahertz to 140
gigahertz. "A typical pulsar would be about 15,000 times fainter at the higher
frequency compared to the lower, and so it wouldn't be detected. It would be
too faint," Camilo said.
In fact,
140 Ghz is the highest frequency ever detected from a neutron star, making XTE
J1810-197 the brightest neutron star known.
Vanishing
Act
Camilo
doesn't expect the light show
to last forever though. Like the X-rays, which have fizzled out since the 2003
outburst, the radio pulses will most likely fade as the star's rotation slows.
"It could
be next month, which is why we've been collecting data with all the telescopes
we can get our hands on, like crazy, in case it disappears quickly, and also
because it's so cool. Or it could be in 100 years," Camilo said.
Do
astronomers expect to find more oddball magnetars? "We shall see," said one of
the co-authors, John Reynolds, officer-in-charge at the CSIRO Parkes
Observatory. "This discovery will certainly heighten interest among pulsar
astronomers throughout the world in observing magnetars. It will be a little
surprising if more are not detected in the coming months."
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