  The emission of the plerion G21.5-0.9 appears more extended in X rays than in
radio. This is an unexpected result because it would imply that short-lived
X-ray electrons may reach distances even larger than radio electrons. Applying
an empirical relationship between dust scattering optical depth and
photoelectric column density, the measured column density leads to a large
optical depth at 1 keV, of about 1. Therefore we investigate the hypothesis
that the detected halo be an effect of dust scattering, re-analyzing an Cal/PV
XMM-Newton observation of G21.5-0.9 and critically examining it in terms of a
dust scattering model. We also present a spectral analysis of a prominent
extended feature in the northern sector of the halo.
