2007ApJ...655..831T

Hydrodynamic simulations predict that low-redshift baryons are predominantly in a warm-hot intergalactic medium (WHIM), which should exhibit absorption and emission lines in the soft X-ray region. The WHIM is predicted to be most dense around clusters of galaxies. We present our XMM-Newton RGS observations of X Comae, an active galactic nucleus behind the Coma Cluster. We detect absorption by Ne IX and O VIII at the redshift of Coma with an equivalent width of 3.3±1.8 and 1.7±1.3 eV, respectively (90% confidence errors or 2.3 and 1.9 σ confidence detections determined from Monte Carlo simulations). The combined significance of both lines is 3.0 σ, again determined from Monte Carlo simulations. The same observation yields a high-statistics EPIC spectrum of the Coma Cluster gas at the position of X Com. We detect emission by Ne IX with a flux of 2.5±1.2x10^–8 photons/cm2/s/arcmin2 (90% confidence errors or 3.4 σ confidence detection). These data permit a number of diagnostics to determine the properties of the material causing the absorption and producing the emission. Although a wide range of properties are permitted, values near the midpoint of the range are temperature ∼4x10⁶ K, density ∼6x10^–6/cm3 corresponding to an overdensity with respect to the mean of ∼32, and line-of-sight path length through it ∼41(Z/Z_☉)^–1 Mpc, where (Z/Z_☉) is the neon metallicity relative to solar. These are properties that have been predicted of the WHIM, so we conclude that we have detected the WHIM associated with the Coma Cluster.