Column density ratios versus N(CO). Lines correspond to models T_∗ = 5 × 10⁴ K and L_X = 0 erg s^-1 (solid lines); T_∗ = 10⁵ K and L_X = 0 erg s^-1 (dashed lines); T_∗ = 3 × 10⁵ K and L_X = 0 erg s^-1 (dotted lines); T_∗ = 10⁵ K and L_X = 10³¹ erg s^-1 (dot-dashed lines); T_∗ = 10⁵ K and L_X = 10³² erg s ⁻ 1 (dot-dot-dashed lines). Other parameters are those from the standard model. Squares and triangles represent observational data, with triangles indicating upper or lower limits, according to their orientation. (A color version of this figure is available in the online journal.)