Zusammenfassung
The unprecedentedly bright optical afterglow of GRB 130606A at a redshift
close to the reionization era (z = 5.913) provides a new opportunity to probe
the ionization status of intergalactic medium (IGM). Here we present a
high-precision analysis of the red Ly alpha damping wing of the afterglow
spectrum taken by Subaru/FOCAS during 10.4-13.2 hr after the burst. We find
that the minimal model including only the baseline power-law and HI 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 in the short wavelength range cannot be explained either by
extinction at the host with standard extinction curves or by the known
systematic uncertainties in the observed spectrum. The red damping wing by
intervening HI gas outside the host can reduce the residual by about 3 sigma
statistical significance. We find that a damped Ly alpha system is not favored
as the origin of this intervening HI absorption, from the observed Ly beta and
metal absorption features. Therefore absorption by diffuse IGM remains as a
plausible explanation. A fit by a simple uniform IGM model requires HI neutral
fraction of f_HI ~ 0.1-0.5 depending on the distance to the GRB host,
implying high f_HI 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. Further investigations using more GRB afterglows to various sightlines,
in comparison with realistic theoretical simulations, are desirable.
Nutzer