X-ray emission from warm-hot intergalactic medium: the role of resonantly scattered cosmic X-ray background

We revisit calculations of the X-ray emission from the warm-hot intergalactic medium (WHIM) with particular focus on contribution from the resonantly scattered cosmic X-ray background (CXB). If the significant part of the CXB emission is resolved into point sources, the properties of the WHIM along the line of sight are recorded in the absorption lines in the stacked spectrum of resolved sources and in the emission lines in the remaining diffuse signal. For the strongest resonant lines, this implies a factor of ∼30 boost in emissivity compared to the intrinsic emissivity over the major part of the density-temperature parameter space region relevant for WHIM. The overall boost for the 0.5-1 keV band is ∼4, declining steeply at temperatures above 10⁶ K and overdensities δ ≳ 100. In addition to the emissivity boost, contribution of the resonant scattering changes relative intensities of the lines, so it should be taken into account when line-ratio diagnostics from high-resolution spectra or redshift determination from low-resolution spectra are considered. Comparison between WHIM signatures in X-ray absorption and emission should allow differentiating truly diffuse gas of small overdensity from denser clumps having small filling factor by future X-ray missions.