Afterglow spectrum of the GRB 191004B covering the LL and Lyα line. The flux of the escaping LyC photons obtained for log(N_H I/cm⁻²) = 17.20±0.15 corresponds to the blue solid and dashed lines below the LL. The orange solid (dashed) line corresponds to the fit of the afterglow continuum without (with) correction for dust extinction. The flux density F_λ is in units of 10⁻¹⁷ erg s⁻¹ cm⁻² Å⁻¹. The spectrum is rebinned to 2 Å per pixel.