2004ApJ...609..785L

At the early, ejecta-dominated stage of supernova remnant (SNR) expansion, a fraction of the swept-up circumstellar magnetic field is dynamically compressed to approximate equipartition at the contact discontinuity separating the SN progenitor's wind (or the interstellar medium) and the ejecta. We propose that the thin nonthermal X-ray filaments observed by the Chandra satellite in several young SNRs are associated with such ``pile-up'' of the magnetic field. We use a one-dimensional diffusion-convection transport equation to describe the propagation of nonthermal electrons near the contact discontinuity of a young SNR and to calculate spatially resolved emission spectra in the X-ray and TeV bands. The results suggest that the high-energy electrons are possibly accelerated at the forward shock and emit efficiently only when they diffuse into regions of high magnetic field near the contact discontinuity. Much more likely, though, is that they are locally accelerated at the contact discontinuity, in which case the acceleration cannot be related to Fermi-type processes and should occur because of other plasma mechanisms. As a consequence, the forward shock in young SNRs is inconspicuous and often unobservable, similar to that in the Crab Nebula.