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The unprecedentedly bright optical afterglow of GRB 130606A located by Swiftat a redshift close to the reionization era (z = 5.913) provides a newopportunity to probe the ionization status of intergalactic medium (IGM). Herewe present an analysis of the red Ly alpha damping wing of the afterglowspectrum taken by Subaru/FOCAS during 10.4-13.2 hr after the burst. We findthat the minimal model including only the baseline power-law and HI absorptionin the host galaxy does not give a good fit, leaving residuals showing concavecurvature in 8400-8900 A with an amplitude of about 0.6% of the flux. Such acurvature in the short wavelength range cannot be explained either byextinction at the host with standard extinction curves, intrinsic curvature ofafterglow spectra, or by the known systematic uncertainties in the observedspectrum. The red damping wing by intervening HI gas outside the host canreduce the residual by about 3 sigma statistical significance. We find that adamped Ly alpha system is not favored as the origin of this intervening HIabsorption, from the observed Ly beta and metal absorption features. Thereforeabsorption by diffuse IGM remains as a plausible explanation. A fit by a simpleuniform IGM model requires HI neutral fraction of f_HI ~ 0.1-0.5 depending onthe distance to the GRB host, implying high f_HI IGM associated with theobserved dark Gunn-Peterson (GP) troughs. This gives a new evidence that thereionization is not yet complete at z ~ 6.

Probing Intergalactic Neutral Hydrogen by the Lyman Alpha Red Damping Wing of Gamma-Ray Burst 130606A Afterglow Spectrum at z = 5.913