Optical vs. X-ray flux for XXL-N (left) and XXL-S (right). The r′ magnitude of the primary counterpart is plotted against the broad-band soft flux [0.5–2 keV]. Symbols and colour-codes are as follows: pink dots correspond to magnitudes available only from either CFHT (preferred) or SDSS (in XXL-N), or either BCS (preferred) or DECam (in XXL-S); green asterisks to stars identified as such by their spectroscopic redshift (star flag 1); red diamonds to stars identified visually (star flag 2); blue asterisks to spectroscopic and visual stars (star flag 3). The dashed and dotted lines mark the AGN locus of X∕O =0 ± 1 (e.g. Brandt & Hasinger 2005).