We propose to study cosmic reionization using absorption line spectra of high-redshift Gamma Ray Burst ( GRB ) afterglows .
We show that the statistics of the dark portions ( gaps ) in GRB absorption spectra represent exquisite tools to discriminate among different reionization models .
We then compute the probability to find the largest gap in a given width range [ W _ { max } ,W _ { max } + dW ] at a flux threshold F _ { th } for burst afterglows at redshifts 6.3 \leq z \leq 6.7 .
We show that different reionization scenarios populate the ( W _ { max } ,F _ { th } ) plane in a very different way , allowing to distinguish among different reionization histories .
We provide here useful plots that allow a very simple and direct comparison between observations and model results .
Finally , we apply our methods to GRB 050904 detected at z = 6.29 .
We show that the observation of this burst strongly favors reionization models which predict a highly ionized intergalactic medium at z \sim 6 , with an estimated mean neutral hydrogen fraction x _ { HI } = 6.4 \pm 0.3 \times 10 ^ { -5 } along the line of sight towards GRB 050904 .