2009ApJ...695.1351B

We present XMM Reflection Grating Spectrometer (RGS) and Chandra Low-Energy Transmission Grating (LETG) observations of the blazar, H 2356-309, located behind the Sculptor Wall, a large-scale galaxy structure expected to harbor high-density Warm-Hot Intergalactic Medium (WHIM). Our simultaneous analysis of the RGS and LETG spectra yields a 3σ detection of the crucial redshifted O VII Kα line with a column density (≳10¹⁶/cm2) consistent with similar large-scale structures produced in cosmological simulations. This represents the first detection of nonlocal WHIM from X-ray absorption studies where XMM and Chandra data are analyzed simultaneously and the absorber redshift is already known, thus providing robust evidence for the expected repository of the "missing baryons."