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SNR and Cosmic Ray Acceleration In a Nutshell
Cosmic rays are extremely high energy particles;
protons and electrons accelerated to nearly the speed of light. They are
over a billion times more energetic than particles created in
accelerators on Earth. They are found everywhere in the Galaxy; millions
of cosmic rays would hit the Earth each day were it not for the
magnetosphere which shields us from them. With current instruments,
however, it is almost impossible to tell what direction the cosmic rays
come from, especially for the really high energy ones. Astronomers have
been puzzling
over exactly where they come from and how they are accelerated for many many
years.
It has long been thought (though there was no direct proof) that cosmic
rays up to a certain energy range are accelerated in the shocks of supernova
remnants. An indirect argument for this mechanism is that the total amount of
energy generated by supernova explosions (and transfered
into kinetic energy of the remnant) is more than enough to account for the
observed cosmic rays. The conclusion is that most,
if not all, of the cosmic rays in the Galaxy below a certain energy are
accelerated in this way.
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Further, synchrotron radiation from SNRs, caused by the acceleration of
energetic particles in a magnetic field, was observed at radio and optical
wavelengths in the 1950s, suggesting the existence
of a population of accelerated particles.
Since synchrotron radiation arises from energetic particles moving in a
magnetic field, the existence of a synchrotron spectrum means that there
must be a magnetic field and a population of accelerated particles in or
near the SNR. From the energy of the synchrotron emission and the
estimated strength of the magnetic field we can calculate the energies
of the population of accelerated particles. Synchrotron radiation was
predicted by the Russian theorist Iosef Shklovsky, but he did not have
access to a telescope large enough to test his hypothesis with
observations. In 1955, Walter Baade, using the new 200-inch telescope on
Mt. Wilson, discovered the polarization of visible light from the Crab
nebula, which demonstrated that emission at optical wavelengths was from
synchrotron radiation.
|  <SN1006: Saga of two spectra |
Thus, there was good circumstantial evidence, but it has not
been until very recently that conclusive evidence supporting this hypothesis has
been found. X-ray spectra of the remnant from SN 1006 taken by ASCA
showed conclusively that the remnant was generating
synchrotron radiation in its
outer rims. The X-ray energies of the discovered synchrotron radiation
accounted for cosmic rays up to 1015eV. The spectrum in the outer
rims
was a straight line while the spectrum of emission from the much fainter
center showed thermal emission from a hot gas. The observations conflicted
with other theories that had been used to explain SNR spectra up until
this time. These observations also demonstrated that particles were
accelerated up to 1015eV, accounting for a significant
portion of galactic cosmic rays.
Do you want to know more?
Thank you to Glenn Allen for contributing to this article.
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