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 tomake accurate measurements of the radio spectral index variations andpolarization. We have compared these new radio images with Halpha, infrared,and X-ray data to better understand the three dimensional structure anddynamics of Kepler, and to better understand the physical relationships betweenthe various nonthermal and thermal plasmas. Spatial variations in the radiospectral index from -0.85 to -0.6 are observed between 6 cm and 20 cm. There isa strong correspondence between the radial and azimuthal profiles of the radio,X-ray, Halpha, and infrared emission in different locations around the remnantalthough there is no single, global pattern. Spectral tomography shows that theflat- and steep-spectrum radio emissions have distinct structures. Theflat-spectrum radio emission is found either at a larger radius than orcoincident with the steep-spectrum emission. We interpret these spectralcomponents as tracing forward- and reverse-shocked material, respectively. TheHalpha and IR images are very similar. Their leading edges are coincident andare either in front of or coincident with the leading edges of the X-ray andradio emission. The X-ray emission matches the Halpha and IR emission inplaces, and in other places traces the steep-spectrum radio emission. In thenorth there is also an anticorrelation in the azimuthal profiles around theremnant of the flat-spectrum radio emission and the thermal X-ray, Halpha, andIR emissions. We suggest that this could be due to a relative weakening of theparticle acceleration at the forward shock due to Alfven wave damping inregions of high density.Comment: accepted to ApJ, 17 figures, 43 page