Measuring the Cosmic-Ray Acceleration Efficiency of a Supernova Remnant

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Science  07 Aug 2009:
Vol. 325, Issue 5941, pp. 719-722
DOI: 10.1126/science.1173383

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Cosmic Shock Waves

Cosmic rays are high-energy charged particles that bombard Earth from all directions in the sky; those originating from within our Galaxy are thought to be accelerated in the shockwaves produced by supernova explosions. Helder et al. (p. 719, published online 25 June; see the Perspective by Raymond) measured the velocity of a section of the blast wave created by supernova RCW 86, an exploding star believed to have been witnessed by Chinese astronomers in 185 A.D., and the post-shock proton temperature. The post-shock proton temperature was much lower than would be expected without any cosmic ray acceleration, which implies that the pressure induced by cosmic ray exceeds the thermal pressure behind the shock.


Cosmic rays are the most energetic particles arriving at Earth. Although most of them are thought to be accelerated by supernova remnants, the details of the acceleration process and its efficiency are not well determined. Here we show that the pressure induced by cosmic rays exceeds the thermal pressure behind the northeast shock of the supernova remnant RCW 86, where the x-ray emission is dominated by synchrotron radiation from ultrarelativistic electrons. We determined the cosmic-ray content from the thermal Doppler broadening measured with optical spectroscopy, combined with a proper-motion study in x-rays. The measured postshock proton temperature, in combination with the shock velocity, does not agree with standard shock heating, implying that >50% of the postshock pressure is produced by cosmic rays.

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