Temperature profile of H₂CO as a function of offset from the MYSO and hot core W3 IRS4 along a slice which passes through the filament. Left: the position of the slice is indicated as a grey line on the temperature map from the  XCLASS fitting of H₂CO (see Sect. 3.2.1 and Fig. 7). The temperature at each position is calculated as an average over the beam FWHM of the map (shown in the lower-left corner, 0.8′′), indicated with the grey dashed lines. This profile for H₂CO (black), and that for CH₃CN (red), are shown in the middle-left panel. The middle-right panel shows a zoom-in focusing on IRS4, with the profile shown in logarithmic units in the right-hand panel. The green dashed lines in both panels indicate three boundaries, between which we perform power-law fits to the temperature profile. The solid magenta line shows the fit over the whole region, while the cyan and blue lines show the fits to only the inner and outer regions. These latter two fits are shown as solid lines in the regions where they are fit and dashed lines where they are extrapolated to the same region as the magenta line. The results of these fits are given in Table 7. The vertical dashed black lines in the middle-left and right-hand panels indicate the beam size. The black contours in the maps show the continuum at the same resolution as the H₂CO data, starting at 5σ and increasing to 45σ in 10σ steps (σ = 0.81 mJy beam⁻¹), while the purple arrow in the middle-right panel indicates the suggested direction of the outflow as in Fig. 9.