  X-ray synchrotron emission tells us of the highest energy reached by
accelerated electrons. In a few supernova remnants (SN 1006, G347.3-0.5) this
is the dominant form of X-ray radiation, but in most it is superposed to the
dominant thermal emission. Thanks to the spectro-imaging capability of Chandra
and XMM-Newton, X-ray synchrotron emission has now been unambiguously detected
in most young supernova remnants (Cas A, Tycho, Kepler). It arises in a very
thin shell (a few arcsecs) at the blast wave. The thinness of that shell (much
broader in the radio domain) implies that the high energy electrons cool down
very fast behind the shock. The magnetic field that one deduces from that
constraint is more than 100 muG behind the shock.
