Probing the Neutral Fraction of the IGM with GRBs during the Epoch of Reionization

Understand

We show that near-infrared observations of the red side of the Ly-alpha line from a single gamma ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM), x_H, at the GRB's redshift to better than ~0.3.

• Some GRB sight-lines will encounter more neutral hydrogen than others at fixed x_H owing to the patchiness of reionisation.
• GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra.
• We discuss the constraints on x_H from the z = 6.3 burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows.