T.R.: Testing reionization with gammaray burst absorption spectra

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 mod-els. We then compute the probability to find the largest gap in a given width range [Wmax, Wmax + dW] at a flux threshold Fth for burst afterglows at redshifts 6.3 ≤ z ≤ 6.7. We show that different reionization scenarios populate the (Wmax, Fth) 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 favours reionization models which predict a highly ionized intergalactic medium at z ∼ 6, with an estimated mean neutral hydrogen fraction xH I = 6.4 ± 0.3 × 10−5 along the line of sight towards GRB 050904. Key words: intergalactic medium – large-scale structure of Universe – gamma-rays: bursts