2025MNRAS.541...29C

Recent observations expanded our understanding of galaxy formation and evolution, yet key challenges persist in the X-ray regime, crucial for studying active galactic nuclei (AGNs). These limitations drive the development of next-generation observatories such as ESA's NewAthena. Now in phase B (preliminary design), the mission requires extensive testing to ensure compliance with its scientific goals, particularly given the uncertainties surrounding high-redshift AGN. This work leverages the ILLUSTRISTNG cosmological simulation to build an X-ray AGN mock catalogue and assess the performance of NewAthena's Wide Field Imager. We created a supermassive black hole (SMBH) light-cone, spanning 10 $\mathrm{deg^2}$, with corrections to account for the limited resolution of the simulation and X-ray properties derived in post-processing. The resulting catalogue reveals a $\sim 5\times$ overabundance of faint AGN compared to current X-ray constraints, an inconsistency potentially resolved by invoking a higher Compton-thick (CTK) fraction and intrinsic X-ray weakness, as suggested by recent JWST findings. An end-to-end survey simulation using SIXTE predicts $\sim$250 000 AGN detections, including $\sim$20 000 at z > 3 and 35 in the epoch of reionization (EoR; z > 6); notably, only AGN with ${L_X} > 10^{43.5}$ erg $s^{-1}$ are detectable at $z > 6$. The analysis also forecasts a significant population of detectable CTK AGN, even beyond z > 4. These findings suggest X-ray observations will, for the first time, probe a significant AGN population in the EoR, offering new insights into SMBH growth. They also provide key input for refining NewAthena's mission design and optimizing its survey strategy.