VLT/X-shooter UVB arm spectrum of GRB 181020A. The absorption features from the H I Lyα, Lyβ and H₂ Lyman-Werner band line transitions are clearly shown. The raw spectrum is shown in gray with the associated error spectrum in blue. A binned version of the spectrum is overplotted in black. A synthetic spectrum of the best fit model to H I and H₂ is shown as the solid red line and the Lyα and Lyβ absorption features are marked at the observed redshift of z = 2.938. Top panel: a zoom-in of a subset of the most constraining Lyman-Werner band line transitions from molecular hydrogen. The redshifted wavelengths of the strongest rotational levels J = 0, 1, 2, 3 are marked above each line.