With NASA's Fermi Gamma-ray Space Telescope, astronomers
now are getting their best look at those whirling stellar cinders
known as pulsars. In two studies published in the July 2, 2009 edition of
Science Express, international teams have analyzed gamma-rays from
two dozen pulsars, including 16 discovered by Fermi. Fermi is the
first spacecraft able to identify pulsars by their gamma-ray emission
alone.
A pulsar is the rapidly spinning and highly magnetized core left
behind when a massive star explodes. Most of the 1, 800 cataloged
pulsars were found through their periodic radio emissions.
Astronomers believe these pulses are caused by narrow,
lighthouse-like radio beams emanating from the pulsar's magnetic
poles.
"Fermi has truly unprecedented power for discovering and studying
gamma-ray pulsars, " said Paul Ray of the Naval Research Laboratory in
Washington. "Since the demise of the Compton Gamma Ray Observatory a
decade ago, we've wondered about the nature of unidentified gamma-ray
sources it detected in our galaxy. These studies from Fermi lift the
veil on many of them."
The Vela pulsar, which spins 11 times a second, is the brightest
persistent source of gamma rays in the sky. Yet gamma rays -- the
most energetic form of light -- are few and far between. Even Fermi's
Large Area Telescope sees only about one gamma-ray photon from Vela
every two minutes.
"That's about one photon for every thousand Vela rotations, " said
Marcus Ziegler, a member of the team reporting on the new pulsars at
the University of California, Santa Cruz. "From the faintest pulsar
we studied, we see only two gamma-ray photons a day."
Radio telescopes on Earth can detect a pulsar easily only if one of
the narrow radio beams happens to swing our way. If not, the pulsar
can remain hidden.
A pulsar's radio beams represent only a few parts per million of its
total power, whereas its gamma rays account for 10 percent or more.
Somehow, pulsars are able to accelerate particles to speeds near that
of light. These particles emit a broad beam of gamma rays as they arc
along curved magnetic field lines.
The new pulsars were discovered as part of a comprehensive search for
periodic gamma-ray fluctuations using five months of Fermi Large Area
Telescope data and new computational techniques.
"Before launch, some predicted Fermi might uncover a handful of new
pulsars during its mission, " Ziegler added. "To discover 16 in its
first five months of operation is really beyond our wildest dreams."
Like spinning tops, pulsars slow down as they lose energy. Eventually,
they spin too slowly to power their characteristic emissions and
become undetectable.
But pair a slowed dormant pulsar with a normal star, and a stream of
stellar matter from the companion can spill onto the pulsar and
increase its spin. At rotation periods between 100 and 1, 000 times a
second, ancient pulsars can resume the activity of their youth. In
the second study, Fermi scientists examined gamma rays from eight of
these "born-again" pulsars, all of which were previously discovered
at radio wavelengths.
"Before Fermi launched, it wasn't clear that pulsars with millisecond
periods could emit gamma rays at all, " said Lucas Guillemot at the
Center for Nuclear Studies in Gradignan, near Bordeaux, France. "Now
we know they do. It's also clear that, despite their differences,
both normal and millisecond pulsars share similar mechanisms for
emitting gamma rays."
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle
physics partnership, developed in collaboration with the U.S.
Department of Energy, along with important contributions from
academic institutions and partners in France, Germany, Italy, Japan,
Sweden, and the U.S.
For images related to this story:
http://www.nasa.gov/mission_pages/GLAST/news/pulsar_passel.html
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