Testing Reionization with Gamma Ray Burst Absorption Spectra

TL;DR

This paper introduces a method to study cosmic reionization by analyzing the absorption spectra of high-redshift Gamma Ray Bursts, providing a new way to distinguish between different reionization models.

Contribution

It demonstrates how dark gap statistics in GRB spectra can differentiate reionization scenarios and applies this method to a specific GRB observation.

Findings

Different reionization models show distinct patterns in the (Wmax, Fth) plane.

Observation of GRB 050904 favors highly ionized intergalactic medium at z~6.

Estimated neutral hydrogen fraction along the line of sight is very low.

Abstract

We propose to study cosmic reionization using absorption line spectra of high-redshift Gamma Ray Burst (GRB) afterglows. We show that the statistics of the dark portions (gaps) in GRB absorption spectra represent exquisite tools to discriminate among different reionization models. We then compute the probability to find the largest gap in a given width range [Wmax, Wmax + dW] at a flux threshold Fth for burst afterglows at redshifts 6.3 < z < 6.7. We show that different reionization scenarios populate the (Wmax, Fth) plane in a very different way, allowing to distinguish among different reionization histories. We provide here useful plots that allow a very simple and direct comparison between observations and model results. Finally, we apply our methods to GRB 050904 detected at z = 6.29. We show that the observation of this burst strongly favors reionization models which predict a highly ionized intergalactic medium at z~6, with an estimated mean neutral hydrogen fraction xHI = 6.4 \pm 0.3 \times 10^-5 along the line of sight towards GRB 050904.