Density of EWS AGN in the 2–10 keV X-ray flux vs H_E observed magnitude parameter space. The Euclid detectable AGN are shown in purple, whilst undetectable AGN are represented in grey. The AGN population detectable with Euclid photometry will, in part, reside beyond the X-ray flux limits of current X-ray surveys. There is also a portion of parameter space where a population of AGN detectable in modern X-ray surveys remain undetectable in the EuclidH_E band. X-ray flux limits for the XMM-COSMOS (black; 90% completeness, F_{2 − 10 keV} = 9.3 × 10⁻¹⁵ erg s⁻¹ cm⁻²; Cappelluti et al. 2009), XXL-3XLSS (orange; 90% completeness, F_{2 − 10 keV} = 3.8 × 10⁻¹⁴ erg s⁻¹ cm⁻²; Chiappetti et al. 2018), the XMM-SERVS W-CDF-S field (blue; 90% of total area, F_{2 − 10 keV} = 2.9 × 10⁻¹⁴ erg s⁻¹ cm⁻²; Ni et al. 2021) surveys, and the XXL 1000 brightest AGN sample (XXL-1000-AGN) as used in Fotopoulou et al. (2016b) for SED fitting (red; sample flux limit, F_{2 − 10 keV} = 4.8 × 10⁻¹⁴ erg s⁻¹ cm⁻²; Fotopoulou et al. 2016b) are plotted for reference. The dotted black lines represent log₁₀(F_X/F_opt) = ± 1. The majority of AGN should lie between these lines.