Symmetry of the radio emission from two high-latitude supernova remnants, G296.5 + 10.0 and G327.6 + 14.6 (SN 1006)

Abstract

We present new high-resolution radio observations at 843 MHz of the shell supernova remnants G296.5+10.0 and G327.6+14.6. The remnants show a high degree of bilateral symmetry, both in their emission intensities and in the outer boundaries of the emission. Both remnants show weak emission and indistinct boundaries near the axes of symmetry. We explore various explanations of the symmetry, which is apparent in both the radio and X-ray maps, and extend an old idea that it is the result of the expansion of a remnant into a region with an orderly alignment of the ambient interstellar magnetic field oriented with a substantial component normal to the line of sight. The observations are consistent with a scenario in which the production of the emitting particles is more effective in shocks propagating transverse to the field lines. This explanation rests upon the fact that measurements of collisionless shocks in the Earth's magnetosphere and in interplanetary space show significant differences between the structures of quasi-parallel and quasi-perpendicular shocks and upon the recent work of Jokipii which shows that the relativistic particle acceleration rate will be a strong function of the angle between the shock normal and the field direction. G296.5+10.0, the more evolved of the two remnants shows a distinctly larger diameter along the symmetry axis than across it. This suggests that under certain conditions, the coupling of the kinetic energy of the shocked material to newly swept-up gas may be incomplete for shocks propagating approximately along field lines. These various effects may also be present in low-latitude remnants but may not be easily discerned because of more disorderly magnetic field in the Galactic plane. On the large scale the X-ray and radio emission show similar distributions in each remnant, but with no detailed correspondence of features. The radio emission from G296.5+10.0 shows some fine structure in the form of filaments aligned tangentially to the shell. We interpret the filaments as multiple sheets or crushed clouds seen on edge, partially compressed and cooled yet still too hot to radiate substantially in the visible.