Probing the warm intergalactic medium through absorption against gamma-ray burst X-ray afterglows

Gamma-ray burst (GRB) afterglows close to their peak intensity are among the brightest X-ray sources in the sky. Despite their fast power-law-like decay, when fluxes are integrated from minutes up to hours after the GRB event, the corresponding number counts (log N-log F relation) far exceed those of any other high-redshift (z > 0.5) source, the flux of which is integrated over the same time interval. We discuss how to use X-ray afterglows of GRBs as distant beacons to probe the warm (10(5) K < T< 10(7) K) intergalactic matter in filaments and outskirts of clusters of galaxies by means of absorption features, the "X-ray forest." According to current cosmological scenarios, this matter may comprise 30%-40% of the baryons in the universe at z < 1. Present-generation X-ray spectrometers such as those on Chandra and XMM-Newton can detect it along most GRBs' lines of sight, provided afterglows are observed soon enough (within hours) after the burst. A dedicated medium-sized X-ray telescope (effective area <less than or similar to>0.1 m(2)) with pointing capabilities similar to that of Swift (minutes) and high spectral resolution (E/DeltaE greater than or similar to 300) would be very well suited to exploit the new diagnostic and study the physical conditions in the universe at the critical moment when structure is being formed.