Reduced spectrum of the QSO J013539.27–212628.4 (z = 4.94, R = 19.55 mag, Li et al. 2018) showing the Lyman-alpha emission and a portion of the Lyman forest. Reduction was carried out at the telescope with the ESPRESSO DRS (see Sect. 3.3.2), and the spectra were resampled by the ESPRESSO DAS (see Sect. 3.3.2, Cupani et al. 2016, 2019) using a fixed log-wavelength grid with a spectral bin of 2 km s⁻¹ (resampled to 10 km s⁻¹ for displaying purposes).