Evolution of the ratio between the flux extrapolated from each mission band assuming Γ = 1.7 and n_H = 3 × 10²⁰ cm⁻², and the true flux of a source, depending on the value of its photon index Γ. The dashed lines correspond to the reference (Γ = 1.7 and ratio of 1), and the dotted lines correspond to a factor of 5. While ROSAT goes over the threshold of 5 for the softest sources, what matters most to our study is that at a given Γ the ratio between different missions is below five (to avoid spurious alerts).