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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