Expected X-ray luminosity function at z ∼ 6.5 (left) and z ∼ 7.5 (right), of AXIS (black) and ATHENA (red), respectively. The Vito et al. (2014) XLFs at the same redshifts are plotted as a blue dashed line and a solid cyan line, respectively. In the right panel, the range of predictions from the hydro-dynamical simulations and semi-analytical models discussed in Amarantidis et al. (2019, khaki area) is also shown for comparison.