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It is known that the GRB equivalent hydrogen column density (NHX) changeswith redshift and that, typically, NHX is greater than the GRB host neutralhydrogen column density. We have compiled a large sample of data for GRB NHXand metallicity [X/H]. The main aims of this paper are to generate improved NHXfor our sample by using actual metallicities, dust corrected where availablefor detections, and for the remaining GRB, a more realistic average intrinsicmetallicity using a standard adjustment from solar. Then, by approximating theGRB host intrinsic hydrogen column density using the measured neutral column(NHI,IC) adjusted for the ionisation fraction, we isolate a more accurateestimate for the intergalactic medium (IGM) contribution. The GRB sample meanmetallicity is = -1.17+/-0.09 rms (or 0.07+/-0.05 Z/Zsol) from a sample of 36GRB with a redshift 1.76 < z < 5.91, substantially lower than the assumption ofsolar metallicity used as standard for many fitted NHX . Lower GRB host meanmetallicity results in increased estimated NHX with the correction scaling withredshift as log (NHX cm-2) = (0.59+/-0.04)log(1+z) + 0.18+/-0.02. Of the 128GRB with data for both NHX and NHI,IC in our sample, only 6 have NHI,IC > NHXwhen revised for realistic metallicity, compared to 32 when solar metallicityis assumed. The lower envelope of the revised NHX - NHI,IC, plotted againstredshift can be fit by log(NHX - NHI,IC cm-2) =20.3 + 2.4 log(1+z). This istaken to be an estimate for the maximum IGM hydrogen column density as afunction of redshift. Using this approach, we estimate an upper limit to thehydrogen density at redshift zero (n0) to be consistent with n0 = 0.17 x10-7cm-3.

monthly notices of the royal astronomical society

Using realistic host galaxy metallicities to improve the GRB X-ray  equivalent total hydrogen column density and constrain the intergalactic  medium density