The glowing Crab Nebula, a spectacular and colorful object
famously imaged by the Hubble Space Telescope, represents the leftovers from a
supernova explosion observed by Chinese and Arab astronomers in 1054 C.E. The
star that exploded is still there, but now in the form of a dense, spinning
pulsar that emits lighthouse beams of radiation.
Now scientists have discovered high-energy gamma rays near
the pulsar that oscillate in alignment with the star's spin. The finding may
shed light on the intense environment deep inside the Crab
Nebula.
When it died, the star that created the Crab Nebula had run
out of fuel to power nuclear reactions, so it collapsed under its own gravity
into the condensed form of a neutron star (these are made only of neutrons — no
protons or electrons). The explosion pushed out the star's outer layers to form
the gassy nebula visible to telescopes today. The neutron star is called a
pulsar because of the pulses of light we see when it spins, and it's encircled
by a swirling web of speeding particles and electromagnetic fields.
"Mainly, we are trying to understand the magnetic
fields around the pulsar and the emission mechanisms that are making the
pulsars pulse," said researcher David J. Clark, an astrophysicist at the
University of Southampton in the U.K. "The gamma rays we found are being
produced by this cloud of high speed particles circling around the
pulsar."
The researchers, led by Anthony Dean of the University of
Southampton, found that the gamma
rays are polarized, meaning their electric fields move up and down in one
direction, which happens to be in the same direction as the axis of the
pulsar's spin. This tells the scientists that the light is produced close to
the pulsar.
The study, funded by the Italian Space Agency and the U.K.'s
Science and Technology Funding Council, is detailed in the Aug. 29 issue of the
journal Science.
The discovery is "yielding insights into the processes
and mechanisms involved in making a dead star so active," wrote Annalisa
Celotti in a commentary article in the same issue of Science. She is an
astrophysicist at the International School for Advanced Studies in Italy, who was
not involved in the research. "The high degree and direction of
polarization at high energies reported by Dean et al. provide valuable
information on the site of acceleration of the particles and on the structure
of the magnetic field associated with the pulsar."
Learning more about the Crab Nebula system helps scientist
unravel the complicated workings of pulsars in
general. The Crab, located about 6,500 light-years from Earth in the
constellation Taurus, is the brightest pulsar around, so it's the easiest to
study.
"This is the brightest object in the gamma ray sky, the
one we have the most data for," Clark told SPACE.com. "The
important thing we were trying to do in the first place is actually measure
polarization, because people haven't been able to measure polarization to a
good degree of accuracy before."
The recent measurements came thanks to the European Space
Agency's International Gamma-Ray Astrophysics Laboratory satellite, which
observes the gamma-ray sky.
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