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Observations with the Swift satellite of X-ray afterglows of more than ahundred gamma ray bursts (GRBs) with known redshift reveal ubiquitous softX-ray absorption. The directly measured optical depth \tau at a given observedenergy is found to be constant on average at redshift z > 2, i.e., <\tau (0.5keV) >_{z > 2} = 0.40+/- 0.02. Such an asymptotic optical depth is expected ifthe foreground diffuse intergalactic medium (IGM) dominates the absorptioneffect, and if the metallicity of the diffuse IGM reaches ~ 0.2 - 0.4 solar atz = 0. To further test the IGM absorption hypothesis, we analyze the 12 highestS/N (> 5000 photon) z > 2 quasar spectra from the XMM-Newton archive, which areall extremely radio loud (RLQs). The quasar optical depths are found to beconsistent with the mean GRB value. The four lowest-z quasars (2 < z < 2.5),however, do not show significant absorption. The best X-ray spectra ofradio-quiet quasars (RQQs) at z > 2 provide only upper limits to theabsorption, which are still consistent with the RLQs, albeit with much lowerS/N (< 1000 photons at z ~ 4). Lack of quasar absorption poses a challenge tothe smooth IGM interpretation, and could allude to the opacity being rather dueto the jets in RLQs and GRBs. However, the jet absorbing column would need toappear in RLQs only at z > 2.5, and in GRBs to strongly increase with z inorder to produce the observed tendency to a constant mean \tau. High X-rayspectral resolution can differentiate between an absorber intrinsic to thesource that produces discernible spectral lines, and the diffuse IGM thatproduces significant absorption, but no discrete features.

Can the soft X-ray opacity towards high redshift sources probe the missing baryons?