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F. Fiore (OAR), F. Nicastro (OAR and SAO), S. Savaglio (OAR and STSci), L. Stella (OAR), M. Vietri (Universita' Roma III)
GRB afterglows close to their peak intensity are among the brightest X-ray sources in the sky. When fluxes are integrated from minutes up to hours after the GRB the corresponding logN-logF exceeds that of any other z>0.5 source. We discuss how to use X-ray afterglows as beacons to probe the warm logT=5-7K IGM in filaments and outskirts of clusters of galaxies by means of absorption features, the ``X-ray forest''. The Chandra and XMM-Newton spectrometers can detect it along most GRBs' lines of sight, provided afterglows are observed within 2-4 hours after the burst. A dedicated medium-sized X-ray telescope with pointing capabilities similar to that of Swift and spectral resolution >300 would be well suited to exploit the new diagnostic and study the physical conditions in the Universe at the critical moment when structure is being formed.