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The extreme brightness of gamma-ray burst (GRB) afterglows and their simplespectral shape make them ideal beacons to study the interstellar medium oftheir host galaxies through absorption line spectroscopy. Using 69low-resolution GRB afterglow spectra, we conduct a study of the rest-frameequivalent width (EW) distribution of features with an average rest-frame EWlarger than 0.5 A. To compare an individual GRB with the sample, we develop EWdiagrams as a graphical tool, and we give a catalogue with diagrams for the 69spectra. We introduce a line strength parameter (LSP) that allows us toquantify the strength of the absorption features as compared to the sample by asingle number. Using the distributions of EWs of single-species features, wederive the distribution of column densities by a curve of growth (CoG) fit. Wefind correlations between the LSP and the extinction of the GRB, the UVbrightness of the host galaxies and the neutral hydrogen column density.However, we see no significant evolution of the LSP with the redshift. There isa weak correlation between the ionisation of the absorbers and the energy ofthe GRB, indicating that, either the GRB event is responsible for part of theionisation, or that galaxies with high-ionisation media produce more energeticGRBs. Spectral features in GRB spectra are, on average, 2.5 times stronger thanthose seen in QSO intervening damped Lyman-alpha (DLA) systems and slightlymore ionised. In particular we find larger excess in the EW of CIV1549 relativeto QSO DLAs, which could be related to an excess of Wolf-Rayet stars in theenvironments of GRBs. From the CoG fitting we obtain an average number ofcomponents in the absorption features of GRBs of 6.00(-1.25,+1.00). The mostextreme ionisation ratios in our sample are found for GRBs with low neutralhydrogen column density, which could be related to ionisation by the GRBemission.

The distribution of equivalent widths in long GRB afterglow spectra