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."
