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