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We use Gamma-ray burst (GRB) spectra total continuum absorption to estimatethe key intergalactic medium (IGM) properties of hydrogen column density($\mathit{N}\textsc{hxigm}$), metallicity, temperature and ionisation parameterover 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. Weuse more realistic host metallicity, dust corrected where available, ingenerating the host absorption model, assuming that the host intrinsic hydrogencolumn density is equal to the measured ionisation corrected intrinsic neutralcolumn from UV spectra ($\textit{N}\textsc{h}\/\ \textsc{i,ic}$). We find thatthe IGM property results are similar, regardless of whether the model assumesall 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 PIEmodel implies a less rapid decline in average metallicity with redshiftcompared 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 contributessubstantially to the total absorption seen in GRB spectra and that thiscontribution rises with redshift, explaining why the hydrogen column densityinferred from X-rays is substantially in excess of the intrinsic hostcontribution measured in UV.

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