Revisiting the Intergalactic Medium Around GRB 130606A and Constraints on the Epoch of Reionization

TL;DR

This paper reanalyzes the high-redshift gamma-ray burst GRB 130606A to better constrain the neutral hydrogen fraction during the Epoch of Reionization, addressing previous conflicting results and emphasizing the importance of model assumptions.

Contribution

It introduces a comprehensive reanalysis using multiple damping wing models, clarifies sources of previous discrepancies, and provides updated upper limits on the neutral fraction at z~6.

Findings

Neutral fraction upper limit xHI < 0.20 to 0.23 at 3-sigma confidence

Differences in model assumptions cause previous conflicting results

Highlights need for more high-redshift GRB observations

Abstract

Gamma-ray bursts (GRBs) are excellent probes of the high-redshift Universe due to their high luminosities and the relatively simple intrinsic spectra of their afterglows. They can be used to estimate the fraction of neutral hydrogen (i.e., neutral fraction) in the intergalactic medium at different redshifts through the examination of their Lyman-alpha damping wing with high quality optical-to-near-infrared spectra. Neutral fraction estimates can help trace the evolution of the Epoch of Reionization, a key era of cosmological history in which the intergalactic medium underwent a phase change from neutral to ionized. We revisit GRB 130606A, a z ~ 5.9 GRB for which multiple analyses, using the same damping wing model and data from different telescopes, found conflicting neutral fraction results. We identify the source of the discrepant results to be differences in assumptions for key damping wing model parameters and data range selections. We perform a new analysis implementing multiple GRB damping wing models and find a 3-sigma neutral fraction upper limit ranging from xHI < 0.20 to xHI < 0.23. We present this result in the context of other neutral fraction estimates and Epoch of Reionization models, discuss the impact of relying on individual GRB lines of sight, and highlight the need for more high-redshift GRBs to effectively constrain the progression of the Epoch of Reionization.