Peak value of a Gaussian fit for the absorption depth T_off−T_on for different methods of estimating T_off. Top and middle panels: method using a polynomial fits with second and forth order, respectively. Bottom panel: method using a spatially averaged off spectrum shown in Fig. 5. The white contours represent the integrated ¹³CO emission at levels of 5 K km s⁻¹ for reference. The circles from left to right marked the same positions as in Fig. 1 where Spectra 1 to 5 shown in Fig. 6 are extracted, respectively.