Plot of hydrogen column density (log N_H) vs. ionization parameter (log U_H), with constraints based on the measured ionic column densities shown in Table 2. Measurements are shown as solid curves, while the dashed curves show lower limits. The colored shades indicate the uncertainties in the constraints of the parameters based on the uncertainties in column density. The black dot shows the solution of log N_H and log U_H that best matches the column densities, while the black ellipse indicates the 1-σ error.