a) Linear polarisation degree P_L map of the compact core region of SNR G21.5-0.9. The map was constructed using polarimetric imaging observations performed with VLT/ISAAC and was smoothed using a 2-dimensional Gaussian with FWHM = 0.45′′. Colour-coded is the measured value of P_L. Contours show the extent of the compact core seen with Chandra HRC (Camilo et al. 2006). b) Map of linearly polarised intensity smoothed with a 2-dimensional Gaussian with a FWHM = 0.45′′. Colour-coded is the measured value of L. White bars represent electric field vectors with their length proportional to the linear polarisation fraction P_L and their orientation defined by the polarisation angle θ. A vertical bar placed in the bottom right corner, for reference, shows an electric field vector with P_L = 1 and θ = 0°. In both panels a star shows the pulsar’s position.