The synchrotron emission observed in plerions depends on the characteristics of the magnetic fields and relativistic electrons in these supernova remnants. Therefore, an analysis of the spectral and spatial properties of this emission, combined with models for the evolution and structure of plerions, would allow one to investigate the evolution of the synchrotron nebula, the structure of the magnetic field, and the distribution of the relativistic electrons, as well as to put constraints on the history of the energy input from the associated neutron star.
The identification of a new class of plerionic remnants, with spectral properties different from the Crab Nebula, has been proposed. The spectra of these objects typically show a sharp spectral break at very low frequencies (below 50 GHz), with a steep spectrum right beyond the break. In order to model the properties of their emission, a nonstandard evolution of the pulsar output seems to be required.
X-ray observations of the synchrotron emission from the Crab Nebula are shown. They are compared with previous data, and their implications on the structure of the Crab are discussed. Recent millimetric data of this object are also presented. A spatially resolved analysis, based on radio, millimetric, and X-ray data, will be carried out also for other plerions.