| Abstract: | Observations with the Swift satellite of X-ray afterglows of more than a hundred gamma ray bursts (GRBs) reveal ubiquitous soft X-ray absorption. The directly measured optical depth at a given observed energy is found to be constant on average at high redshift z > 2. Such an asymptotic optical depth is expected if the foreground diffuse intergalactic medium (IGM) dominates the absorption effect, and if the metallicity of the diffuse IGM is 0.2 − 0.4 solar at z = 0. Testing the IGM absorption hypothesis against a high-z quasar sample yields mixed results. Four radio loud quasars (RLQs) at 2 < z < 2.5 do not show significant absorption and the few available radio quiet quasars give only upper limits. Lack of quasar absorption poses a challenge to the IGM interpretation, and could allude to the opacity being rather due to the jets in RLQs and GRBs. Finally, a method for unambiguously distinguishing between IGM and host absorption will be described. |
