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B.M. Gaensler (MIT / U Sydney / ATNF), A.J. Green (U Sydney), G.M. Dubner, E.B. Giacani (IAFE), W.M. Goss (NRAO)
Even from the earliest crude radio maps of 40 years ago, it was apparent that Galactic supernova remnants (SNR) are anything but well-behaved expanding spheres. In particular a significant fraction of SNRs have a bilateral, or ``barrel'', morphology, with a clear axis of reflection symmetry along which there is negligible emission, and with bright flanks on either side. There are numerous theories to explain the appearance of barrel-shaped SNRs, such as the effect of anisotropies in the distribution of supernova ejecta, the structure of the ambient magnetic field, the mass-loss history of the progenitor and the effect of opposed jets or beams from a central source. Three decades of debate have failed to settle the issue, however.
Most Galactic SNRs have now been observed in the radio with high resolution and sensitivity, and one can establish a clear bilateral subset of the population. We define such a subset, and find that the symmetry axes of these SNRs are aligned with the Galactic Plane at a high level of statistical significance. This suggests that the Galactic magnetic field is responsible for the appearance of these SNRs at some level. We propose that the ambient field pre-processes the interstellar medium to produce elongated bubbles into which these SNRs expand. This then produces the barrel shape, together with the required alignment. We are in the process of studying the environment of barrel-shaped SNRs in H\,{\sc i}\ emission, in order to see if this model is supported by observations.
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The author(s) of this abstract have provided an email address for comments about the abstract: bmg@space.mit.edu