The nature of the X-ray halo of the plerion G21.5–0.9 unveiled by XMM-Newton and Chandra F. Bocchino, E. van der Swaluw, R. Chevalier and R. Bandiera A&A, 442 2 (2005) 539-548 Published online: 07 October 2005 DOI: 10.1051/0004-6361:20052870