continuously. The observations were carried out in double-sideband (DSB) using the wide band spectrometer (WBS), whose resolution is $\sim 1.1$MHz.

For the present study we have employed bands 1a (488–552GHz), 2b (724–792GHz), and 4a (967–1042GHz), corresponding to the IDs number 1342191499, 1342192332, and 1342191619, respectively. HSA provides level 2.5 data products obtained with HIFI pipeline of the *Herschel* interactive processing environment [HIPE, @ott2010HIPE]. These data generally offer a scientific quality sufficient to perform the line analysis. However, given the weakness of the and spectral signals, we have also gone through the final processing manually. Starting from level 2 products, we have inspected all the data, manually removed bad scans, and exported all the “good” 1GHz chunks to CLASS/GILDAS[^4] format.

Side-band deconvolution has been performed with the minimization algorithm of @comito2004decon implemented into CLASS90, and the resulting spectra have been cross-checked with the original level 2.5 products. In a few instances, the deconvolution procedure produced a small increase of the noise level compared to the averaged level 2 scans. In such cases, we directly use the DSB spectra after verifying that no interfering emission was present in the image band. At the later stage, intensive baseline subtraction on H- and V-polarisation 