Context. About half of the baryons in the local Universe are invisible and – according to simulations – their dominant fraction resides in filaments connecting clusters of galaxies in the form of low density gas with temperatures in the range of 105 <T <107 K. This warm-hot intergalactic medium has never been detected indisputably using X-ray observations. Aims. We aim to probe the low gas densities expected in the large-scale structure filaments by observing a filament connecting the massive clusters of galaxies A 222 and A 223 (z = 0.21), which has a favorable orientation approximately along our line-of-sight. This filament has been previously detected using weak lensing data and as an over-density of colour-selected galaxies. Methods. We analyse X-ray images and spectra obtained from a deep observation (144 ks) of A 222/223 with XMM-Newton. Results. We present observational evidence of X-ray emission from the filament connecting the two clusters. We detect the filament in the wavelet-decomposed soft-band (0.5–2.0 keV) X-ray image with a 5σ significance. Following the emission down to the 3σ significance level, the observed filament is ≈1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is k T = 0.91±0.25 keV, and its emission measure corresponds to a baryon density of (3.4±1.3)×10−5(l/15 Mpc)−1/2 cm−3, where l is the length of the filament along the line-of-sight. This density corresponds to a baryon over-density of ρ/ ρC ≈ 150. The properties of the gas in the filament are consistent with results of simulations of the densest and hottest parts of the warm-hot intergalactic medium
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