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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.

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.

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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.

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Thank you to Glenn Allen for contributing to this article.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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