Was the Universe neutral beyond redshift six?

We provide measurements of the neutral hydrogen fraction x_HI at z ∼ 6, by comparing semi-analytical models of the Lyα forest with observations of high-z quasars and gamma-ray bursts absorption spectra. We analyze the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ zem ≤ 6.42 studying separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with our models, we conclude that x _HI evolves smoothly from 10^-4.4 at z = 5.3 to 10 ^-4.2 at z = 5.6, with a robust upper limit x_HI < 0.36 at z = 6.3. We show the results of the first-ever detected transverse proximity effect in the HI Lyα forest, produced by the HII region of the faint quasar RD J1148+5253 at z = 5.70 intervening along the LOS of SDSS J1148+5251 at z = 6.42. Moreover, we propose a novel method to study cosmic reionization using absorption line spectra of high-redshift GRBs afterglows. We show that the time evolution and the statistics of gaps in the observed spectra represent exquisite tools to discriminate among different reionization models. By applying our methods to GRB 050904 detected at z = 6.29, we show that the observation of this burst provides strong indications of a highly ionized intergalactic medium at z ∼ 6, with an estimated mean neutral hydrogen fraction x_HI = 6.4 ± 0.3 × 10^-5 along that line of sight.