2005A&A...442..539B

The nature of the radio-quiet X-ray halo around the plerionic supernova remnant G21.5-0.9 is under debate. On the basis of spatial and spectral analysis of a large Chandra and XMM-Newton dataset of this source, we have developed a self-consistent scenario which explains all the observational features. We found that the halo is composed by diffuse extended emission due to dust scattering of X-rays from the plerion, by a bright limb which traces particle acceleration in the fast forward shock of the remnant, and by a bright spot (the ``North Spur'') which may be a knot of ejecta in adiabatic expansion. By applying a model of interaction between the PWN, the remnant and the supernova environment, we argue that G21.5-0.9 progenitor may be of Type IIP or Ib/Ic, and that the remnant may be young (200-1000yr).