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Oct.
17, 2008: About three times a second, a 10,000-year-old
stellar corpse sweeps a beam of gamma-rays toward Earth. Just
discovered by NASA's Fermi Gamma-ray Space Telescope, the
object, called a pulsar, is the first one known that "blinks"
in pure gamma rays.
 "This
is the first example of a new class of pulsars," says
Stanford University's Peter Michelson, principal investigator
for Fermi's Large Area Telescope. "[We think] it will
give us fundamental insights into how these collapsed stars
work."
Right:
An artist's concept of the newly discovered pulsar. Clouds
of charged particles move along the pulsar's magnetic field
lines (blue) and create a lighthouse-like beam of gamma rays
(purple). [Larger
image]
Pulsars
were first discovered
in 1967 by student radio astronomer Jocelyn Bell and her thesis
advisor Tony Hewish. The radio pulses they recorded were uncannily
steady--so much so that some astronomers wondered if they
were picking up signals from extraterrestrial civilizations.
The correct explanation was even stranger: Pulsars are spinning
neutron stars packing the mass of the sun into a sphere about
20 km across. Whirling around thousands of times each hour,
they beam radio pulses into the cosmos in the style of a rapidfire
lighthouse.
Since
then, about 1800 pulsars have been discovered mainly via their
radio emission. A fraction of pulsars go beyond radio; they
also emit pulses of visible light, X-rays, and even high-energy
gamma-rays. This discovery by Fermi is different because it
is a purely gamma-ray pulsar. The star is silent across parts
of electromagnetic spectrum where pulsars are normally found
and hints at a whole population of previously unsuspected pulsars
waiting to be picked out of the heavens.
The
gamma-ray-only pulsar lies within a supernova remnant known
as CTA 1 located about 4,600 light-years away in the constellation
Cepheus. Its lighthouse-like beam sweeps Earth's way every
316.86 milliseconds. The pulsar, which formed in a supernova
explosion about 10,000 years ago, emits 1,000 times the energy
of our sun.
"The
Large Area Telescope provides us with a unique probe of the
galaxy's pulsar population, revealing objects we would not
otherwise even know exist," says Fermi project scientist
Steve Ritz of the Goddard Space Flight Center.
Above:
The pulsar is not located at the center of the surrounding
supernova remnant CTA 1. Click on the image to view a larger
map.
The
pulsar in CTA 1 is not located at the center of the supernova's
expanding gaseous shell. Supernova explosions can be asymmetrical,
often imparting a "kick" that sends the neutron
star careening through space. Based on the remnant's age and
the pulsar's distance from its center, astronomers believe
the neutron star is moving at about a million miles per hour
-- a typical speed for neutron stars.
Fermi's
Large Area Telescope scans the entire sky every three hours
and detects photons with energies ranging from 20 million
to more than 300 billion times the energy of visible light.
"This
observation shows the power of the Large Area Telescope,"
Michelson adds. "It is so sensitive that we can now discover
new types of objects just by observing their gamma-ray emissions."
A
paper about the new pulsar appears in the Oct. 16 edition
of Science Express.
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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