ASA's Fermi Gamma-ray Space Telescope has found 12 previously
unknown pulsars (orange). Fermi also detected gamma-ray emissions
from known radio pulsars (magenta, cyan) and from known or suspected
gamma-ray pulsars identified by NASA's now-defunct Compton Gamma-Ray
Observatory (green). Credit: NASA/Fermi/LAT Collaboration
Since
their discovery 40 years ago, pulsars the rapidly spinning,
highly magnetized crushed cores of exploded stars have largely
been detected via the pulsing radio signals emitted by their
lighthouse beam-like jets. But astronomers have suspected that
these pulses give only the slightest hint of the true power of
these cosmic dynamos.
With
the launch of NASA's Fermi
Gamma-ray Space Telescope (formerly
GLAST) in June of last year, scientists are finally getting a
glimpse of the powerful hearts of these stellar beasts. In its
first four months of operation,
Fermi detected more than three dozen pulsars, 12 of which are
new gamma-ray-only
pulsars.
"We
know of 1,800 pulsars, but until Fermi we saw only little wisps
of energy from all but a handful of them," said pulsar
astronomer Roger Romani of Stanford University in California.
"Now, for dozens of pulsars, we're seeing the actual power of
these machines."
The
astronomers now find that some pulsar bursts are generated far
above the star's surface.
Romani and his colleagues presented the Fermi findings earlier
this month at the annual meeting of the American Astronomical
Society in Long Beach, Calif. Romani said that these were just
the "first wave of such discoveries," and that they will usher
in "a new era of high-energy pulsar physics."
Sweeping beams
Pulsars (short for "pulsating star") were first discovered in
1967 by Jocelyn Bell Burnell and Anthony Hewish. The astronomers
were perplexed by the incredibly regular radio emissions they
detected; they first thought they could be transmissions from
extraterrestrial civilizations.
Astronomers later determined that pulsars were actually rapidly
spinning neutron
stars, the extremely dense and heavy crushed cores left behind
when a massive star explodes.
The
pulsing radio signals from the stars are thought to be caused by
narrow, lighthouse-like beams emanating from the stars' magnetic
poles. If a star's spin axis doesn't align exactly with its
magnetic poles, these beams sweep across the sky. The pulses can
repeat in anywhere from a few milliseconds to a few seconds.
If
one of those beams happens to swing in Earth's direction,
astronomers can detect the signal with radio telescopes.
Unfortunately that makes any census of pulsars automatically
biased because it can't count the pulsars that don't send their
beams our way.
"That
has colored our understanding of neutron stars for 40 years,"
Romani said.
And
while radio beams are easy to detect, they account for only a
tiny fraction (a few parts in a million) of a pulsar's total
power. Gamma rays, on the other hand, account for 10 percent or
more. That's where Fermi comes in.
"For
the first time, Fermi is giving us an independent look at what
heavy stars do," Romani said.
Gamma ray revisions
A
pulsar's intense electric and magnetic fields and rapid spin
accelerate particles to close to the speed
of light.
Gamma rays let astronomers glimpse the particle accelerator's
heart.
Fermi's observations of gamma rays in new and previously known
pulsars is forcing astronomers to revise their picture of how
these particle accelerators work and how the gamma rays are
emitted.
"We
used to think the gamma rays emerged near the neutron star's
surface from the polar cap, where the radio beams form," said
astrophysicist Alice Harding of NASA's Goddard Space Flight
Center in Greenbelt, Md. She helped present the findings. "The
new gamma-ray-only pulsars put that idea to rest."
The
picture that is now emerging is one of pulsed gamma rays arising
far above the neutron star. For the
Vela pulsar,
the brightest persistent gamma-ray source in the sky, the
emission region is thought to lie about 300 miles from the star,
which itself has a diameter of just 20 miles.
Particles produce the gamma rays as they accelerate along arcs
of the pulsar's open magnetic field. This model means that gamma
rays would be beamed broadly across the sky, not in the narrow
beam like the radio signals.
This
finding "puts the nail in the coffin of the classic polar cap
model," Romani said.
There
is still some debate as to whether gamma ray emission starts at
high altitudes over the star or from the star's surface and
emanates all the way out.
"So
far, Fermi observations to date cannot distinguish which of
these models is correct," Harding said.
Millisecond pulsars
Fermi
also picked up pulsed gamma rays from seven "millisecond
pulsars," so called because they spin between 100 and 1,000
times a second.
These
rapid rotators, even by pulsar standards, move at up to one
tenth of the speed of light.
Far
older than "normal" pulsars such as the Vela pulsar, these speed
demons seem to break the rules because pulsars tend to slow in
their rotation as they age (the 10,000-year-old CTA 1 pulsar,
which the Fermi team announced in October, slows by about a
second every 87,000 years).
Millisecond pulsars seem to get a second lease on life because
they reside in binary star systems. As their companion star ages
and expands, it does so to the point where the pulsar can start
to siphon off material, forming an accretion disk. The accretion
disk has force that speeds up the pulsar, allowing it to rotate
"at this furious rate for about a billion years tomorrow,"
Harding said.
These
discoveries are likely just the beginning of what Fermi will
find in terms of nearby pulsars, Harding said. "We probably will
be discovering a lot more."
