Smoothed (Gaussian kernel with 0.01° sigma), exposure-corrected images of diffuse X-ray emission in two energy bands (as indicated in the figures). The color scale is linear between the threshold, as defined in the text, and the maximum pixel intensity. The 90 cm radio contours of G 15.4+0.1 are overplotted (blue). The dashed lines indicate the regions used for spectral analysis. The cross (orange) indicates the best-fit position of HESS J1818−154.