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The observed synchrotron brightness of sources such as radio galaxies and supernova remnants depends on the number density of relativistic electrons, their energy distribution, the distribution of magnetic field strengths and the observing frequency. We have developed a technique that can both remove the frequency dependence of the synchrotron spectrum and express the sychrotron brightness simply in terms of the total number of particles along the line-of-sight and the perpendicular component of the magnetic field. Similarly, the observedspectral index can be expressed in terms of the perpendicular componentof the magnetic field and a fiducial relativistic electron energy. Using this method, we can isolate the above physical parameters.
By applying our method to VLA observations of the powerful radio galaxy, Cygnus A (Carilli et al. 1991), we find that the variations in the continuum surface brightness are primarily due to variations in thenumber of relativistic electrons along our line-of-sight. We also identify a prominant new feature in the eastern lobe.
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