Probing intergalactic neutral hydrogen by the Lyman alpha red damping wing of gamma-ray burst 130606A afterglow spectrum at z = 5.913

The unprecedentedly bright optical afterglow of GRB 130606A located by Swift at a redshift close to the reion- ization era (z = 5.913) provides a new opportunity to probe the ionization status of intergalactic medium (IGM). Here we present an analysis of the red Lydamping wing of the afterglow spectrum taken by Subaru/FOCAS dur- ing 10.4-13.2 hr after the burst. We find that the minimal mode l including only the baseline power-law and H I absorption in the host galaxy does not give a good fit, leaving residuals showing concave curvature in 8400-8900 ˚ A with an amplitude of about 0.6% of the flux. Such a curvature i n the short wavelength range cannot be explained either by extinction at the host with standard extinction cu rves, intrinsic curvature of afterglow spectra, or by the known systematic uncertainties in the observed spectrum. The red damping wing by intervening H I gas outside the host can reduce the residual by about 3� statistical significance. We find that a damped Lysystem is not favored as the origin of this intervening H I absorption, from the observed Lyand metal absorption features. Therefore absorption by diffuse IGM remains as a plausible explanation. A fit by a simple uniform IGM model requires H I neutral fraction of fH I � 0.1-0.5 depending on the distance to the GRB host, implying high fH I IGM associated with the observed dark Gunn-Peterson (GP) troughs. This gives a new evidence that the reionization is not yet complete at z � 6.