Upper panel: spectrum of the X-ray afterglow of GRB 120711A convolved with a Gaussian as described in the text (black solid line). The best-fit model is shown by the green line. The dashed lines are the percentile curves obtained with the simulations for emission and absorption lines. They correspond to single-trial detection significance of 2, 3, and 4σ. Lower panel: same curves of the upper panel, normalised by dividing by the best-fit model.