Spectral Index Variations and Physical Conditions in Kepler ’

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 Hα, 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. The mean spectral index is −0.71. The mean percent polarization is 3.5% at 20 cm and 6% at 6 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Hα, and infrared emission in different locations around the remnant although there is no single, global pattern. Spectral tomography shows that the flatand 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 forwardand reverse-shocked material, respectively. The flat-spectrum radio emission can alternatively be interpreted as the bow-shocked material (reshocked by the forward shock) from the progenitor’s motion through the interstellar medium. The Hα 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 Hα 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