Detection of thermal X-ray emission in the halo of the plerionic supernova remnant G21.5-0.9

The detection of a soft thermal X-ray component in the spectrum of a bright knot in the halo of the plerion G21.5-0.9 is reported. Using a collisional ionization equilibrium model for an hot optically thin plasma, a temperature of kT 0.12-0.24 keV, a mass of 0.3-1.0 M⊙ and a density of 1.6-6 cm−3 is derived. The spectral analysis suggests a possible overabundance of Silicon with respect to the solar value in the knot; if this will be confirmed this object may be a clump of shocked ejecta.