studied with the Chandra X-ray Observatory [@vas96; @kas01; @rob03 and references therein]. The PWN is elongated along the NE-SW direction with a total extent of $1'$, and appears to be surrounded by a radio synchrotron nebula with similar extent and shape. The distance to G11.2$-$0.3 determined from absorption is 5 kpc [@gre88]. The overall morphology of G11.2$-$0.3 resembles Cassiopeia A (Cas A). Both have a thick, bright, and clumpy shell, although the shell of Cas A is much brighter than that of 11p2, i.e., 2720 Jy vs 22 Jy at 1 GHz [@gre04] [^1]. At a distance of 3.4 kpc, the outer radius of Cas A shell is 2.0 pc and its expansion velocity is 4,000–6,500   [e.g., @fes96], while they are 2.9 pc and $\sim 1,000$  for G11.2$-$0.3. Cas A has a faint $5'$ (or 2.5 pc)-diameter plateau extending beyond the bright shell [e.g., @hwa04]. The plateau represents swept-up circumstellar (or ambient) medium while the bright shell is thought to be mainly the ejecta swept up by a reverse shock. Such plateau has not been detected in 11p2. [@che05] classified both Cas A and G11.2$-$0.3 into SN IIL/b category which has a red supergiant (RSG) progenitor star with some H 