Neutral Fraction of Hydrogen in the Intergalactic Medium Surrounding High-Redshift Gamma-Ray Burst 210905A
arxiv(2024)
摘要
The Epoch of Reionization (EoR) is a key period of cosmological history in
which the intergalactic medium (IGM) underwent a major phase change from being
neutral to almost completely ionized. Gamma-ray bursts (GRBs) are luminous and
unique probes of their environments that can be used to study the timeline for
the progression of the EoR. Here we present a detailed analysis of the ESO Very
Large Telescope X-shooter spectrum of GRB 210905A, which resides at a redshift
of z 6.3. We focus on estimating the fraction of neutral hydrogen, xHI, on
the line of sight to the host galaxy of GRB 210905A by fitting the shape of the
Lyman-alpha damping wing of the afterglow spectrum. The X-shooter spectrum has
a high signal to noise ratio, but the complex velocity structure of the host
galaxy limits the precision of our conclusions. The statistically preferred
model suggests a low neutral fraction with an 3-sigma upper limit of xHI <
0.15, indicating that the IGM around the GRB host galaxy is mostly ionized. We
discuss complications in current analyses and potential avenues for future
studies of the progression of the EoR and its evolution with redshift.
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