Bottom: C¹⁸O(2–1) integrated intensity map of the simulation (Clarke et al. 2018), with the black square indicating the region that is studied in this figure. The red box indicates the region displayed in Fig. 12. Left: ¹²CO(4–3) and C¹⁸O(2–1) spectra extracted from the square which show a blueshifted ¹²CO(4–3) component (blue) in addition to the component related to the bulk emission of the filament crest (red). The two velocity intervals used for the plot on the right are indicated in blue and red. Right: physical location along the line of sight of the gas in the velocity intervals, showing that the larger blueshifted velocity interval covers a small region compared to the filament and that it is located at the border of this filament where the density strongly increases because of a filament accretion shock.