Dependence of physical parameters on the adopted ionizing spectrum for the observed low-redshift system at z = 1.7529. The wide and narrow marked areas in the upper, middle, and lower panels show the ionization parameter log U, metallicity [Si/H], and relative abundance [Si/C], respectively, versus the parameters of the ionizing SED (Δ₁, Δ₂, Δ₃; from the left to the right) for all best-fit models and those consistent with the observed column density ratios of Al ii/Al iii and Si ii/Si iii on a 5σ level, respectively.