0.5–2 keV luminosity as a function of redshift for the z > 3 sources detected in the simulated AXIS (black circles) and ATHENA (red squares) surveys. 1σ uncertainties are computed using the Gehrels (1986) equations. The z > 5.5 AGNs detected in the X-rays by currently available facilities are also plot for comparison (Vito et al. 2019, blue stars; Nanni et al. 2017, cyan diamonds). Two different models of black hole seed accretion are also shown (light SMBH seed: dashed yellow line; heavy SMBH seed: solid magenta line; see the text for more details).