Best-fitting number Q_ion of ionizing photons at the illuminated face of the absorbing cloud of gas (divided by the quantity 4πc), vs. the best-fitting ionization parameter, U, for the three flux-interval spectra HS, MS and LS. Both the x and y-axis are in logarithmic scale. The straight line indicates the best-fitting linear relationship between log(Q_ion) and log (U), whose intercept provides an estimate for the quantity n_eR² (see Sect. 4.2).