High-Resolution VLA Imaging of the Supernova Remnant W28 at 328 and 1415 MHz

We present new VLA images at 328 and 1415 MHz of the supernova remnant (SNR) W28. The image at 1415 MHz, produced after the combination of 50 separate VLA pointings and the addition of single-dish data, has an angular resolution of 88'' × 48'', with an rms noise of 5 mJy beam^-1. At 328 MHz, the image has an angular resolution of 97'' × 52''  and an rms noise of 14 mJy beam^-1. From these observations we reestimated the characteristic parameters of W28: angular diameter 48', flux densities S_{1415 MHz} = 246 Jy and S_{328 MHz} = 425 Jy, Σ_{1 GHz}∼1.8 × 10^-20 W m^-2Hz^-1 sr^-1, and α = -0.35 (S ∝ ν^α). The radio spectrum has local variations which correlate with total intensity features. The brighter radio filaments have a spectral index systematically flatter than the rest. The comparison of the radio emission with images in other spectral ranges reveal several excellent correlations. We conclude that (1) the encounter of the SNR shock with a molecular cloud to the east of W28 produced enhanced synchrotron and thermal X-ray emission, compression and distortion in the radio shell, Hα filaments, and the excitation of many OH (1720 MHz) masers exactly located in the interface between the shock front and the eastern molecular cloud; and (2) the interaction of the shock front moving toward us with molecular gas located in front of it has been responsible for the formation of the bright radio filaments seen in projection in the northern half of W28 and the excitation of more OH masers. Also, excellent radio/Hα correspondence is observed toward the northwest of the SNR. The total energy in relativistic particles is estimated in 2 × 10⁴⁷ ergs.