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The unprecedentedly bright afterglow of Swift GRB 130606A at z = 5.91 gave usa unique opportunity to probe the reionization era by high precision analysesof the redward damping wing of Ly alpha absorption, but the reportedconstraints on the neutral hydrogen fraction (f_HI) in intergalactic medium(IGM) derived from spectra taken by different telescopes are in contradiction.Here we examine the origin of this discrepancy by analyzing the spectrum takenby VLT with our own analysis code previously used to fit the Subaru spectrum.Though the VLT team reported no evidence for IGM HI using the VLT spectrum, weconfirmed our previous result of preferring non-zero IGM HI (the best-fit f_HI~ 0.06, when IGM HI extends to the GRB redshift). The fit residuals of the VLTspectrum by the model without IGM HI show the same systematic trend as theSubaru spectrum. We consider that the likely origin of the discrepancy betweenthe two teams is the difference of the wavelength ranges adopted in thefittings; our wavelength range is wider than that of the VLT team, and also weavoided the shortest wavelength range of deep Ly alpha absorption (lambda_obs <8426 A), because this region is dominated by HI in the host galaxy and thesystematic uncertainty about host HI velocity distribution is large. We alsostudy the sensitivity of these results to the adopted Ly alpha cross sectionformulae, ranging from the classical Lorentzian function to the most recent onetaking into account fully quantum mechanical scattering. It is found that thepreference for non-zero IGM HI is robust against the choice of the crosssection formulae, but it is quantitatively not negligible and hence one shouldbe careful in future analyses.

publications of the astronomical society of japan

High Precision Analyses of Lyman alpha Damping Wing of Gamma-Ray Bursts  in the Reionization Era: On the Controversial Results from GRB 130606A at z =  5.91