New clues
that cosmic rays, high-energy particles that travel space and bombard the
Earth, are generated by shock
waves in supernova remnants were revealed by a new study using NASA's Chandra
X-ray observatory.
Cosmic
rays are composed of high-energy electrons, protons and ions. Scientists
used the Chandra observatory to study the X-rays
emitted by the electrons (the only one of the particles that emits X-rays) from
cosmic rays emanating from Cassiopeia
A, a 325-year-old supernova remnant.
Scientists
have long theorized that the high-energy shock waves of exploded stars called supernovas,
were among "the few places in the galaxy
that have enough energy to accelerate these particles," said Michael Stage, an
astronomer at the University of Massachusetts, Amherst.
They
theorize that the particles, contained by magnetic fields on either side of the
shock wave, bounce back and forth across the shock, eventually energizing the
electrons to very high energies.
"The
electrons pick up speed each time they bounce across the shock front, like
they're in a relativistic pinball machine," said Glenn Allen, a team member
from the Massachusetts Institute of Technology. "The magnetic fields are like
the bumpers and the shock is like a flipper."
One
important key for this theory to work, Stage says, is that the acceleration of
the particles should come close to the theoretical maximum rate. The images of
X-ray radiation from Cassiopeia A allowed the scientists to map out the
accelerations of the electrons and provided "direct evidence that [this
maximum] is reached for electrons," Stage told SPACE.com.
Stage says
that it is likely that protons and ions would be accelerated in the same way as
the electrons.
By looking
at the X-ray images, scientists also observed that the highest-energy electrons
accelerated very quickly to their energies, whereas lower energy electrons took
much longer to accelerate.
Acceleration
of charged particles also occurs in shocks in the Earth's magnetosphere
and in jets
produced by supermassive
black holes.
"Explaining
where cosmic rays come from helps us to understand other mysterious phenomena
in the high-energy universe," Stage said.
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