X-ray synchrotron emission from supernova remnants

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 SNRs (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 around 100 microGauss behind the shock. In SN 1006, the bipolar emission structure can now be studied in more detail. The near absence of synchrotron X-ray emission toward the center of the remnant favors a geometry in which the bright limbs are polar caps rather than a limb-brightened equatorial belt. This indicates that acceleration is favored where the magnetic field was originally parallel to the shock velocity.