Probing the physical properties of the intergalactic medium using gamma-ray bursts

TL;DR

This study uses gamma-ray burst spectra to estimate intergalactic medium properties like hydrogen density, metallicity, and temperature across redshifts 1.6 to 6.3, revealing how these properties evolve and contribute to observed absorption.

Contribution

It introduces a method combining photo-ionisation and collisional ionisation models with realistic host metallicity corrections to probe IGM properties from GRB spectra.

Findings

IGM hydrogen column density scales as (1+z)^{1.0-1.9}

Metallicity decreases from ~0.1Zsun at z~2 to ~0.001Zsun at z>4

IGM absorption significantly increases with redshift, affecting X-ray observations

Abstract

We use Gamma-ray burst (GRB) spectra total continuum absorption to estimate the key intergalactic medium (IGM) properties of hydrogen column density ($\mathit{N}\textsc{hxigm}$), metallicity, temperature and ionisation parameter over a redshift range of $1.6 \leq z \leq 6.3$, using photo-ionisation (PIE) and collisional ionisation equilibrium (CIE) models for the ionised plasma. We use more realistic host metallicity, dust corrected where available, in generating the host absorption model, assuming that the host intrinsic hydrogen column density is equal to the measured ionisation corrected intrinsic neutral column from UV spectra ($\textit{N}\textsc{h}\/\ \textsc{i,ic}$). We find that the IGM property results are similar, regardless of whether the model assumes all PIE or CIE. The $\mathit{N}\textsc{hxigm}$ scales as $(1 + z)^{1.0\/\ -\/\ 1.9}$, with equivalent hydrogen mean density at $z = 0$ of $n_0 = 1.8^{+1.5}_{-1.2} \times 10^{-7}$ cm$^{-3}$. The metallicity ranges from $\sim0.1Z\sun$ at $z \sim 2$ to $\sim0.001Z\sun$ at redshift $z > 4$. The PIE model implies a less rapid decline in average metallicity with redshift compared to CIE. Under CIE, the temperature ranges between $5.0 <$ log$(T/$K$)<\/\ 7.1$. For PIE the ionisation parameter ranges between $0.1 <$ log$(\xi) < 2.9$. Using our model, we conclude that the IGM contributes substantially to the total absorption seen in GRB spectra and that this contribution rises with redshift, explaining why the hydrogen column density inferred from X-rays is substantially in excess of the intrinsic host contribution measured in UV.