CO(1−0)-normalized SLED fits of QSO PJ231−20. Gold circles indicate the CO luminosity ratios measured in the quasar. The dot-dashed black line is the best-fit model using a single XDR model. The composite (PDR+XDR) model described in the text is represented with the solid black line. The dashed blue and red lines show the PDR and XDR component, respectively. The shadowed area indicates the 1σ uncertainties on the normalization values. Best-fit parameters are reported in the legends at the bottom of the figure. The dashed gray line is the theoretical trend (∝J³) of the CO SLED in the LTE, optically thick, and high-temperature (or low frequency) limit. The red and blue diamonds (slightly shifted horizontally for clarity) are the CO(1−0)-normalized SLED in the PJ231−20 companion galaxy and QSO PJ308−21, respectively. For the PJ231−20 companion we report the CO SLEDs obtained assuming T_dust = 47 K (half-top filled magenta diamonds) and 35 K (half-bottom filled magenta diamonds, see Sect. 8.2).