Probing early cosmic magnetic fields through pair echoes from high-redshift GRBs

We discuss the expected properties of pair echo emission from gamma-ray bursts (GRBs) at high redshifts (z greater than or similar to 5), their detectability, and the consequent implications for probing intergalactic magnetic fields (IGMFs) at early epochs. Pair echoes comprise inverse Compton emission by secondary electron-positron pairs produced via interactions between primary gamma-rays from the GRB and low-energy photons of the diffuse intergalactic radiation, arriving with a time delay that depends on the nature of the intervening IGMFs. At sufficiently high z, the IGMFs are unlikely to have been significantly contaminated by astrophysical outflows, and the relevant intergalactic radiation may be dominated by the well-understood cosmic microwave background (CMB). Pair echoes from luminous GRBs at z similar to 5-10 may be detectable by future facilities such as the Cherenkov Telescope Array or the Advanced Gamma-ray Imaging System, as long as the GRB primary emission extends to multi-TeV energies, the comoving IGMFs at these redshifts are B similar to 10(-16) to 10(-15) G, and the non-CMB component of the diffuse intergalactic radiation is relatively low. Observations of pair echoes from high-z GRBs can provide a unique, in-situ probe of weak IGMFs during the epochs of early structure formation and cosmic reionization.