Resonant scattering of X-rays by the warm intergalactic medium

For the low-density filamentary and sheet-like structures in the warm (simi lar to 10(4) to similar to 10(6) K) intergalactic medium (IGM) predicted by numerical simulations, the resonant line scattering of X-ray background (X RB) photons by He and H-like ions of heavy elements can exceed the 'local' thermal emission by a factor of a few or more. As a result of the conservat ive nature of scattering, this resonantly scattered radiation can only be i dentified if a significant fraction of the XRB is resolved and removed. Whi le the combined spectrum of the resolved sources will contain X-ray absorpt ion features, the residual background will contain corresponding emission f eatures with the same intensity. At the relevant densities and temperatures the lines of He and H-like oxygen at 0.57 and 0.65 keV are most promising. These lines (which have a typical width of similar to1-2 eV) may contain u p to 50 per cent of the total 0.5-1 keV emission of the filament. For a nea rby (z less than or similar to 0.1) filament with a Thomson optical depth o f 10(-4), XMM should detect about 200 photons in the O VII Line during a 10 (5)-s exposure if the metallicity of the gas is as large as observed in gal axy clusters. On average up to a few per cent of the soft XRB could be reso nantly scattered by this phase of the IGM and resonantly scattered photons should account for a significant fraction of the truly diffuse background a t low energies. Close to bright X-ray sources Like galaxy clusters or activ e galactic nuclei (AGN) the flux of scattered radiation will be further enh anced. Off-line blazars are the most promising illuminating sources. The sc attered emission from AGN may also constrain the duration of the active pha se of these objects.