Physics
Scientific paper
Feb 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006sgrb.confe..65v&link_type=abstract
Presented at the KITP Conference: Supernova and Gamma-Ray Burst Remnants, Feb 9, 2006, Kavli Institute for Theoretical Physics,
Physics
Scientific paper
Due to Chandra's capabilities for imaging spectroscopy with very highspatial resolution, it has become clear over the last five years thatvirtually all young supernova remnants emit X-ray synchrotron radiationfrom a region close to the shock front. The X-ray synchrotron radiation isconfined to a region close to the shock front, because the X-ray radiationcomes from the highest energy electrons for which radiation loss timescales are short. The width of the X-ray synchrotron region varies, for example it is small(few arcsec) for Cas A and Tycho, but quite extended for RCW 86 andSN1006. These widths have been interpreted in two ways, both of which canbe used to obtain an estimate of the downstream magnetic fieldstrength: 1) The widths corresponds to the time scale over which theadvected electrons lose so much energy that they stop emitting X-rayradiation (the advection length scale); i.e. width = v_p *t_x, with v_p theplasma velocity and t_xthe X-ray loss time scale. 2) Alternatively, one can assume that the widthscorrespond the the diffusion length scale. For the advection length scaleone has to assume a plasma velocity based on the (sometimes measured) shockvelocity, usually v_p = 1/4v_s, but high shock compression may changethis. For the diffusion length scale method one has to assume a diffusioncoefficient; usually the diffusion is assumed to be at the Bohmlimit, which is the fastest diffusion possible. However, I will show thatfor the highest electron energiesin the loss limited case the two differentlength scales should be roughly equal. This is in agreement with the factthat both methods give similar values for the magnetic field strength nearthe shock front, and that the spectra are steep. However, the insistency ofthe results also suggests that the assumptions made, i.e. Bohmdiffusion, are indeed valid. The high magnetic field strength estimated in conjunction with the evidencefor Bohm diffusion indicates that young supernova remnants must be capableof acceleration ions well above the cosmic ray "knee" (3x10^15eV). Moreover, the rough scaling of B with the density and shock velocitysuggests that the highest energies are reached early in the life of a SNRand preferentially inshocks moving through a red supergiant wind. Finally, I will show evidence that a simple extrapolation of the radiosynchrotron spectrum including an exponential cut-off cannot describe theX-ray synchrotron spectra of Cas A and RCW 86. Instead a flattening of thespectrum has to be assumed in agreement with non-linear shock accelerationtheory.
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