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X-ray emission from young supernova remnants results from two processes: the bulk heating of ions, primarily, at the blast wave, and the subsequent heating of electrons due either to Coulomb collisions with hot ions or to (faster) collective processes such as plasma instabilities. The detailed X-ray image of a SNR can shed light on the processes by which electrons are heated, but also can simply locate the shock wave, or at least provide a lower limit to the shock radius. Radio-bright remnants must be producing new energetic electrons and/or magnetic field behind the shock wave, but the mechanisms are unclear. We exhibit ROSAT PSPC and HRI images of two of the ten highest radio-surface-brightness SNRs in the Galaxy, G11.2--0.3 and 3C 397, and compare them to radio images. The correspondence is excellent. Images in both wavelength regions have unresolved edges at about $1'$ resolution suggesting that the enhanced synchrotron emissivity is produced immediately at the shock front. Azimuthal brightness variations in the shells are correlated in the two wavelength regions, suggesting that both thermal-electron heating and relativistic-electron acceleration occur through processes sensitive to the same properties of the external medium, such as the obliquity angle between the local shock normal and the upstream magnetic field. We also discuss other features of the X-ray observations.
