Radiative transfer predictions and the observed absolute intensity of ¹³CO J = 4 → 3 for T_kin = 160 K. The three red-dashed contours show the observed intensity and its value scaled up by factors of 8 and 16, respectively. The blue-dashed lines show the range of column and volume densities constrained by the analysis of the CO line ratios. The absolute intensity of ¹³CO J = 4 → 3 needs to be scaled up by a factor of about 16 to reproduce the constrained column and volume densities.