BIPOLAR SUPERNOVA-REMNANTS AND THE OBLIQUITY DEPENDENCE OF SHOCK ACCELERATION

Young, adiabatic-phase shell supernova remnants (SNRs) generate a radio synchrotron flux which implies a greater energy density in relativistic particles or magnetic field than can be accounted for by shock compression of ambient cosmic rays and magnetic field. It is likely that diffusive shock acceleration produces the observed relativistic electrons in at least some young remnants. We have generated synthetic radio maps of model SNRs based on several classes of assumptions regarding both shock acceleration and magnetic field evolution. We have not considered turbulent amplification of magnetic field. For these models to replicate the pronounced bipolarity in observed remnants, we find that the efficiency of the acceleration process must depend upon the obliquity angle θBn between the shock normal and the magnetic field (assumed uniform). We calculate azimuthal intensity ratios to enable a quantitative comparison with observations. Isotropic acceleration models cannot produce ratios as large as observed in five of a sample of nine young remnants. In our simple parameterization, "quasi-perpendicular" (θBn ∼ 90°) models can produce observed ratios but predict a smaller median ratio than seen in the sample, and, for one remnant, they are only marginally consistent with observations. However, for small angles between the external magnetic field and the line of sight, "quasi-parallel " models (θBn ∼ 0°) produce images unlike any observed remnants and predict a much larger median ratio than seen in the sample. Our version of "quasi-parallel" acceleration is distinctly less favored by observations.