We provide measurements of the neutral hydrogen fraction x _ { HI } at z \sim 6 , by comparing semi-analytical models of the Ly \alpha 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 \leq z _ { em } \leq 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 Ly \alpha 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 \sim 6 , with an estimated mean neutral hydrogen fraction x _ { HI } = 6.4 \pm 0.3 \times 10 ^ { -5 } along that line of sight . PACS 98.62.Ra - Intergalactic matter ; quasar absorption systems ; Lyman forest PACS 98.54.Aj - Quasars PACS 98.70.Rz - \gamma -ray bursts