Gallerani 2007 Testing Reionization with spectra
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Testing Reionization with Gamma Ray Burst Absorption Spectra
(Submitted on 5 Oct 2007)
Abstract: We propose a novel method to study cosmic reionization using absorption line spectra of high-redshift Gamma Ray Burst (GRB) afterglows. We show that the time evolution and the statistics of the dark portions (gaps) in the observed spectra taken during the first days after the GRB explosion represent exquisite tools to discriminate among different reionization models. We then compute the probability to find the largest gap in a given width range for burst afterglows of observed J-band flux F_J, and redshift z_GRB. We show that different reionization scenarios populate the (F_J,z_GRB) 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 GRB050904 detected at z=6.29 whose largest gap is found to be almost equal to 65 angstrom. 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=7.0 \pm 4.0 10^-4 along the line of sight towards GRB050904.
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