Top: integrated line intensities as a function of E_u for the positions A, B, and C. The lines indicate the best fits of the observed integrated line intensities with RADEX: red for n_H = 10⁶ cm^-3, blue for n_H = 10⁵ cm^-3, and green for n_H = 10⁴ cm^-3. Bottom: optical depth τ calculated with RADEX versus E_u for the positions A, B, and C. : thin symbols, solid line, : thick symbols, dashed line. The results are summarised in Table 2.