Probing the physical properties of the intergalactic medium using
  blazars
  We use Swift blazar spectra to estimate the key intergalactic medium (IGM)
properties of hydrogen column density(Nhxigm), metallicity and temperature over
a redshift range of 0.03 leq z leq 4.7, using a collisional ionisation
equilibrium(CIE) model for the ionised plasma. We adopted a conservative
approach to the blazar continuum model given its intrinsic variability and use
a range of power law models. We subjected our results to a number of tests and
found that the Nhxigm parameter was robust with respect to individual exposure
data and co-added spectra for each source, and between Swift and XMM-Newton
source data. We also found no relation between Nhxigm and variations in source
flux or intrinsic power laws. Though some objects may have a bulk
Comptonisation component which could mimic absorption, it did not alter our
overall results. The Nhxigm from the combined blazar sample scales
as(1+z)^1.8+\-0.2. The mean hydrogen density at z = 0 is n0 = (3.2+\-0.5) x
10^-7 cm^-3. The mean IGM temperature over the full redshift range is log(T\K)
= 6.1+\-0.1 and the mean metallicity is [X\H] = -1.62+\-0.04 (Z sim0.02) When
combining with the results with a gamma-ray burst (GRB) sample, we find the
results are consistent over an extended redshift range of 0.03 leq z leq 6.3.
Using our model for blazars and GRBs, we conclude that the IGM contributes
substantially to the total absorption seen in both blazar and GRB spectra.
