2000ApJ...544L...7F

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 (logN-logF 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 (10⁵K<T<10⁷K) 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 ≲0.1m²) with pointing capabilities similar to that of Swift (minutes) and high spectral resolution (E/ΔE≳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.