IRAC images of the central region of SNR G21.5-0.9: 3.6 μm – top left, 4.5 μm – top right, 5.8 μm – bottom left and 8.0 μm – bottom right. All panels cover the same field of view. Contours in the 4.5 μm image show the extent of the compact nebula as seen with Chandra HRC (Camilo et al. 2006). The star marks the position of PSR J1833-1034. In the top left panel circles depict: large central circle – flux extraction region for the compact nebula; smaller circles: regions where the background level was estimated. These regions were used for measuring the flux in all IRAC bands (see Sect. 3.2). Each image was smoothed with a 2-dimensional Gaussian with FWHM = 1.8′′.