ESA Science & Technology - Publication Archive

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 exceeds that 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 (10⁵ K7 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 fast enough (within hours) after the burst.