2025ApJ...985...28F

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., the neutral fraction) in the intergalactic medium (IGM) at different redshifts through the examination of their Lyα damping wing with high-quality optical-to-near-infrared spectra. Neutral fraction estimates can help trace the evolution of the Epoch of Reionization (EoR), a key era of cosmological history in which the IGM 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 3σ neutral fraction upper limit ranging from ${x}_{{{\rm{H}}}_{{\rm{I}}}}\lesssim 0.20$ to ${x}_{{{\rm{H}}}_{{\rm{I}}}}\lesssim 0.23$. We present this result in the context of other neutral fraction estimates and EoR 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 EoR.