X-ray and radio images of four supernova remnants with three distinct x-ray signatures

New radio images of Kepler's supernova remnant (SNR) and new x-ray images of the three SNRs - PKS1209-52, W28, and 3C400.2 - were studied. Although all of them are shell-like SNRs at radio wavelengths, there are three distinct x-ray signatures among them. New high resolution radio images of Kepler's SNR reveal shell structure which is very similar to that of the x-ray image. Linear polarization mapping at 6 cm shows the existence of a radial component in the magnetic field. There is a correlation between rotation measure and x-ray surface brightness, which makes it possible to estimate a total magnetic field of 7 x 10-5 gauss and an energy density of relativistic electrons ∼ 6 x 10-10 ergs cm-3 in the shell. The correlation between radio emissivity and x-ray emissivity can be ascribed to acceleration of thermal electrons to relativistic energies at the shock front. The X-ray image of PKS1209-52 reveals fragmented shell structure which resembles the radio image. There is, however, a compact X-ray source near its center. The age of PKS1209-52 is in the range 10,000-23,000 years depending on the nature of the ambient medium. The SNRs W28 and 3C400.2 have centrally-peaked x-ray emission despite their radioshell morphologies. Comparisons of their x-ray and radio properties with known crab-like SNRs suggest that the synchrotron radiation process is not a very plausible x-ray production mechanism