The number of detected baryons in the universe at z < 0:5 is much smaller than predicted by standard big bang nucleosynthesis and by the detailed observation of the Ly forest at redshift z ¼ 2. Hydrodynamic simulations indicate that a large fraction of the baryons is expected to be in a ''warm-hot'' (10 5 Y10 7 K) filamentary gas, distributed in the intergalactic medium. This gas, if it exists, should be observable only in the soft X-ray and UV bands. Using the predictions of a particular hydrodynamic model, we simulated the X-ray flux as a function of energy in the 0.1Y2 keV band due to the warm-hot intergalactic medium (WHIM ) and compared it with the flux from other diffuse components. Our results show that as much as 20% of the total diffuse X-ray background ( DXB) in the energy range 0.37Y0.925 keV could be due to X-rays from the WHIM, 70% of which from filaments at redshift between 0.1 and 0.6. Simulations done using a FOVof 3 0 show that in more than 20% of the observations we expect the WHIM flux to contribute to more than 20% of the DXB. These simulations also show that in about 10% of all the observations a single bright filament in the FOVaccounts alone for more than 20% of the DXB flux. Redshifted oxygen lines should be clearly visible in these observations. We also investigate the expected angular distribution of the X-ray flux from the WHIM and found a characteristics angular scale of a few arcminutes. Subject headingg s: diffuse radiation -large-scale structure of universe -X-rays: diffuse background
