SIMBAD references

It is known that the GRB equivalent hydrogen column density (N_HX) changes with redshift and that, typically, N_HX is greater than the GRB host neutral hydrogen column density. We have compiled a large sample of data for GRB N_HX and metallicity [X/H]. The main aims of this paper are to generate improved N_HX for our sample by using actual metallicities, dust corrected where available for detections, and for the remaining GRB, a more realistic average intrinsic metallicity using a standard adjustment from solar. Then, by approximating the GRB host intrinsic hydrogen column density using the measured neutral column (N_HI, IC_) adjusted for the ionization fraction, we isolate a more accurate estimate for the intergalactic medium (IGM) contribution. The GRB sample mean metallicity is = -1.17 ± 0.09 rms (or 0.07 ± 0.05 Z/Z_sol) from a sample of 36 GRB with a redshift 1.76 <= z <= 5.91, substantially lower than the assumption of solar metallicity used as standard for many fitted N_HX. Lower GRB host mean metallicity results in increased estimated N_HX with the correction scaling with redshift as Δlog (N_HX cm^–2) = (0.59 ± 0.04)log(1 + z) + 0.18 ± 0.02. Of the 128 GRB with data for both N_HX and N_HI, IC_ in our sample, only six have N_HI, IC_ > N_HX when revised for realistic metallicity, compared to 32 when solar metallicity is assumed. The lower envelope of the revised N_HX - N_HI, IC_, plotted against redshift can be fit by log(N_HX - N_HI, IC_ cm^–2) = 20.3 + 2.4 log(1 + z). This is taken to be an estimate for the maximum IGM hydrogen column density as a function of redshift. Using this approach, we estimate an upper limit to the hydrogen density at redshift zero (n₀) to be consistent with n₀ = 0.17 x 10^–7cm^–3.