VLT/X-shooter optical and NIR afterglow spectrum of GRB 210905A at redshift z = 6.3. Top left and right: low-ionization absorption lines of the GRB host galaxy system. Here and in the following panels, data are in blue, the fit is in cyan, the error spectrum is in orange, and the vertical green dashed lines indicate the center of the components. Bottom left: fine-structure absorption lines. The top panel shows the Si IIλ1260 Å absorption lines corresponding to the ground-level, as reference. Bottom right: high-ionization absorption lines of the GRB host galaxy system. We stress that Si IV and C IV lines are noisy because they are affected by sky-line residuals and telluric absorptions. The λ1242 Å absorption of N V is blended with the Fe IIλ2734 Å line of the z = 2.8296 foreground absorber. All the plots are in the velocity space and 0 has been fixed arbitrarily to z = 6.3168 (see Sect. 4.3), corresponding to the N Vλ1238 Å absorption line (and to the stronger high-ion ionization line component).