Radio Emission from the Composite Supernova Remnant G326.3–1.8 (MSH 15–56)

High-resolution radio observations of the composite supernova remnant (SNR) G326.3-1.8, or MSH 15-56, with the Australia Telescope Compact Array show details of both the shell and the bright plerion, which is offset about one-third of the distance from the center of the SNR to the shell. The shell appears to be composed of thin filaments, typical of older shell SNRs. The central part of the elongated plerion is composed of a bundle of parallel ridges that bulge out at the ends and form a distinct ring structure on the northwestern end. The magnetic field, with a strength of order 45 μG, is directed along the axis of the ridges but circles around the northwestern ring. This plerion is large and bright in the radio but is not detected in X-ray or optical wavelengths. There is, however, a faint hard X-ray feature closer to the shell outside the plerion. Perhaps, if the supernova explosion left a rapidly moving magnetar with large energy input but initially rapid decay of both relativistic particles and magnetic field, the observed differences with wavelength could be explained.