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

Gamma-ray burst (GRB) afterglows close to their peak intensity are among th e brightest X-ray sources in the sky. Despite their fast power-law-like dec ay, 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 o f 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 GRB s 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 abso rption features, the "X-ray forest." According to current cosmological scen arios, 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 X MM-Newton can detect it along most GRBs' lines of sight, provided afterglow s are observed soon enough (within hours) after the burst. A dedicated medi um-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 spe ctral resolution (E/DeltaE greater than or similar to 300) would be very we ll suited to exploit the new diagnostic and study the physical conditions i n the universe at the critical moment when structure is being formed.