Soft (0.5–2 keV) X-ray flux versus optical (i-band) magnitude for the dropouts (grey points). Blue squares represent the sources with photo-z (peak) higher than 3.5. The green triangles (red circles) show the sources with z_spec ≥ 3.5 (z_spec < 3.5). The solid line indicates the log(F_X/F_opt) = 0, and the dashed lines from left to right correspond to log(F_X/F_opt) = − 2, −1, +1, respectively. The X-ray fluxes are plotted in logarithmic scale and given in units of ergs cm⁻² s⁻¹, while the optical magnitudes are in the AB system. The dashed and dotted contours show the high-z samples of Georgakakis et al. (2015) and Marchesi et al. (2016), respectively.