Sketch of ART-XC (dashed lines) and eROSITA (solid lines) astrophysical and internal detector backgrounds. The blue lines show the estimated sky background using the models of Lumb et al. (2002) and Gruber et al. (1999), convolved with the telescopes’ responses averaged over the FoV. The red lines show the level of the detector internal background (fluorescent lines are not shown). For eROSITA, the astrophysical background dominates below ~ 2 keV, while at higher energies, the detector background exceeds the sky background by an order of magnitude.