Discovery of Small X-ray Ring around Crab Pulsar
Solves Old Mystery
Recent images obtained with the Chandra X-ray Observatory have
uncovered part of the secrets of the Crab Nebula by revealing a bright
small ring around its center. Scientists had long searched for the
missing link between the central
pulsar and the nebula
and it would seem that their quest is now over.
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NASA/CXO/SAO
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"It has never been seen before," a jubilant Jeff Hester, an associate
professor at Arizona State University, said at the press conference
where he was presenting the discovery, "It should tell us a lot about
how the energy from the pulsar gets into the nebula." Hester is one
of the world experts on the Crab Nebula and the leader of a monitoring
research project carried using both the Hubble Space Telescope and the
Chandra X-ray Observatory.
The Crab Nebula is the remnant of a stellar explosion which occurred
at the beginning of the millennium. The event, which was recorded by
Chinese and Japanese astronomers, produced a pulsar (a fast rotating
neutron star) and a bright nebula around it. The system has been
studied intensively -- Hester joked that every astronomer had at least
once used it in their research -- but several mysteries still
remained.
The neutron star in the
Crab Nebula was formed when a massive star,
running out of its nuclear fuel, collapsed under the pressure of its
own gravity. With less than
40 miles circumference at the equator, it would be easy to bike around
neutron stars if it were not for their surface gravity of 200,000,000,000
times that of the Earth. Neutron stars can rotate very rapidly and
because of their large surface magnetic fields, they act as efficient
particle accelerators.
A spinning neutron star releasing a beam of photons and particles acts like a
giant light house. Each time the beam's path crosses that of the Earth,
astronomers detect an increase of incoming
radiation.
This is why these objects are called "pulsars". It is not known if
all neutron stars are pulsars.
One of the remaining mysteries about the Crab was the way the energy
was transferred from the pulsar to the nebula. Astronomers knew that
particles were accelerated by the pulsar and injected into the nebula,
but they didn't know how. "It's a lot like a waterfall hitting some
rocks at the bottom and splattering in all directions," said Hester in
a phone interview. The
electrons are accelerated by
the pulsar, moving together at speeds up to 99.999% that of
light. Hester
surmises that the place where they get splattered into the nebula is
the small bright ring close to the pulsar which appears on the new
Chandra image - a cosmic
equivalent of the rocks at the bottom of the waterfall. "It is the most
likely candidate," says Hester, "Because
it is sitting in the right place and it stands out in the high-energy
image." The other spectacular
features of the Chandra image (the large ring and the
jets) had been detected
during observations with the ROSAT satellite.
The Chandra X-ray Observatory is the latest of NASA's great
observatories project. It was launched this past summer and has an
expected life time of five to ten years. The observatory was named
after the late Subrahmanyan Chandrasekhar, a professor at the university
of Chicago who pioneered many of the ideas about neutron stars.
Chandra also means "moon" in Sanskrit.
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