Radio Spectral Index Variations and Physical conditions in Kepler's   Supernova Remnant
  A new epoch of VLA measurements of Kepler's supernova remnant was obtained to make accurate measurements of the radio spectral index variations and polarization. We have compared these new radio images with Halpha, infrared, and X-ray data to better understand the three dimensional structure and dynamics of Kepler, and to better understand the physical relationships between the various nonthermal and thermal plasmas. Spatial variations in the radio spectral index from -0.85 to -0.6 are observed between 6 cm and 20 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Halpha, and infrared emission in different locations around the remnant although there is no single, global pattern. Spectral tomography shows that the flat- and steep-spectrum radio emissions have distinct structures. The flat-spectrum radio emission is found either at a larger radius than or coincident with the steep-spectrum emission. We interpret these spectral components as tracing forward- and reverse-shocked material, respectively. The Halpha and IR images are very similar. Their leading edges are coincident and are either in front of or coincident with the leading edges of the X-ray and radio emission. The X-ray emission matches the Halpha and IR emission in places, and in other places traces the steep-spectrum radio emission. In the north there is also an anticorrelation in the azimuthal profiles around the remnant of the flat-spectrum radio emission and the thermal X-ray, Halpha, and IR emissions. We suggest that this could be due to a relative weakening of the particle acceleration at the forward shock due to Alfven wave damping in regions of high density. 
Tracey DeLaney, Barron Koralesky, Lawrence Rudnick, John R. Dickel