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 ($mathit {N}_{mathrm{HXIGM}}$), metallicity, temperature, and ionization parameter over a redshift range of 1.6 ≤ z ≤ 6.3, using photoionization equilibrium (PIE) and collisional ionization equilibrium (CIE) models for the ionized 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 ionization corrected intrinsic neutral column from UV spectra (${it N}_{mathrm{H},{small I,IC}}$). We find that the IGM property results are similar, regardless of whether the model assumes all PIE or CIE. The $mathit {N}_{mathrm{HXIGM}}$ scales as (1 + z)1.0-1.9, with equivalent hydrogen mean density at z = 0 of $n_0 = 1.8^{+1.5}_{-1.2} imes 10^{-7}$ cm-3. The metallicity ranges from $sim 0.1, mathrm{Z}_{odot }$ at redshift z ∼2 to $sim 0.001, mathrm{Z}_{odot }$ 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 ionization parameter ranges between 0.1 < log (ζ) < 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.