Proxy for the α_ox index for XXL-N (left panel) and XXL-S (right panel) as a function of optical flux. The optical-to-X-ray spectral index is defined as α_ox = log(f_x∕f_o)∕log(ν_x∕ν_o) where the optical monochromatic flux is computed from the r′ AB magnitude 2.5logf_o = −r′− 48.6, and the reference frequency ν_o derived fromthe λ_eff of the filter (6258Å for CFHT, 6185Å for SDSS, 6266Å for BCS, and 6433Å for DECam); ν_x corresponds to 2 keV; the monochromatic 2 keV flux f_x is extrapolated from the observed [0.5–2 keV] soft flux F_soft using the spectral model of Table 2, i.e. f_x = 7.626176 × 10⁻²⁷ × 2 × 1.47115 × 10⁸F_soft. Only sources detected in the soft X-ray band with a primary counterpart in the r′ filter are considered. Symbols and colour-codes are as in Fig. 7.