Signs of WHIM in the soft X-ray background
  Small angular scale structure of the soft X-ray background correlated with the galaxy distribution is investigated. An extensive data sample from the ROSAT and XMM-Newton archives are used. Excess emission below 1 keV extending up to at least 1.5 Mpc around galaxies is detected. The relative amplitude of the excess emission in the 0.3-0.5 keV band amounts to 1.3+/-0.2 % of the total background flux. A steep spectrum of the emission at higher energies is indicated by a conspicuous decline of the signal above 1 keV. The XMM-Newton EPIC/MOS data covering wider energy range than the ROSAT PSPC are consistent with a thermal bremsstrahlung spectrum with k}T < 0.5 keV. This value is consistent with temperatures of the Warm-Hot Intergalactic Medium derived by several groups from hydrodynamic simulations. Correlation analysis allows for estimate of the average excess emission associated with galaxies but the data are insufficient to constrain physical parameters of the WHIM and to determine the contribution of WHIM to the total baryonic mass density. 
A. M. Soltan, M. J. Freyberg, G. Hasinger