Relativistic electron populations in Cassiopeia A

Spectral index variations are observed in the radio brightness distribution in the supernova remnant Cassiopeia A, between observing wavelengths of 20 and 6 cm. Reliable variations are seen over a range of -0.64 to -0.92, with steeper spectra typically associated with compact features such as the 'bow shocks' and flatter spectra with the bright radio ring. The spatial scales over which significant variations are detected range from about 11 arcsec to greater than 80 arcsec. These results indicate a direct link between the dynamics of the remnant and the relativistic particle energy distributions. Building on previously suggested dynamical pictures, it is suggested that the bow shocks are caused by moderately dense clumps of ejecta newly encountering the shocked ISM. It is found that models based on first-order Fermi acceleration in the test particle limit are unable to explain, concurrently, the steep spectral indices and the high synchrotron emissivities observed in Cas A. Thus, consideration of other types of processes, such as acceleration in cosmic-ray-mediated shocks and turbulent acceleration with secularly increasing B fields, is required.