Schedule Feb 09, 2006
X-Ray Synchrotron Emission and Magnetic Fields in SNRs
Jacco Vink (SRON)

Due to Chandra's capabilities for imaging spectroscopy with very high spatial resolution, it has become clear over the last five years that virtually all young supernova remnants emit X-ray synchrotron radiation from a region close to the shock front. The X-ray synchrotron radiation is confined to a region close to the shock front, because the X-ray radiation comes from the highest energy electrons for which radiation loss time scales 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 and SN1006. These widths have been interpreted in two ways, both of which can be used to obtain an estimate of the downstream magnetic field strength: 1) The widths corresponds to the time scale over which the advected electrons lose so much energy that they stop emitting X-ray radiation (the advection length scale); i.e. width = v_p *t_x, with v_p the plasma velocity and t_xthe X-ray loss time scale. 2) Alternatively, one can assume that the widths correspond the the diffusion length scale. For the advection length scale one has to assume a plasma velocity based on the (sometimes measured) shock velocity, usually v_p = 1/4v_s, but high shock compression may change this.
For the diffusion length scale method one has to assume a diffusion coefficient; usually the diffusion is assumed to be at the Bohm limit, which is the fastest diffusion possible. However, I will show that for the highest electron energiesin the loss limited case the two different length scales should be roughly equal. This is in agreement with the fact that both methods give similar values for the magnetic field strength near the shock front, and that the spectra are steep. However, the insistency of the results also suggests that the assumptions made, i.e. Bohm diffusion, are indeed valid.
The high magnetic field strength estimated in conjunction with the evidence for Bohm diffusion indicates that young supernova remnants must be capable of acceleration ions well above the cosmic ray \"knee\" (3x10^15 eV). Moreover, the rough scaling of B with the density and shock velocity suggests that the highest energies are reached early in the life of a SNR and preferentially inshocks moving through a red supergiant wind.
Finally, I will show evidence that a simple extrapolation of the radio synchrotron spectrum including an exponential cut-off cannot describe the X-ray synchrotron spectra of Cas A and RCW 86. Instead a flattening of the spectrum has to be assumed in agreement with non-linear shock acceleration theory.

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