Resonant scattering of X-rays by the warm intergalactic medium
  For the low density filamentary and sheet-like structures in the warm ($\sim 10^4$ to $\sim 10^6$ K) IGM predicted by numerical simulations the resonant line scattering of X-ray background (XRB) photons by He and H-like ions of heavy elements can exceed the ``local'' thermal emission by a factor of a few or more. Due to the conservative nature of scattering this resonantly scattered radiation can only be identified if a significant fraction of the XRB is resolved and removed. While the combined spectrum of the resolved sources will contain X-ray absorption features, the residual background will contain corresponding emission features 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 $\sim$ 1-2 eV) may contain up to 50% of the total 0.5-1 keV emission of the filament. For a nearby ($z \la 0.1$) filament with a Thomson optical depth of $10^{-4}$ XMM should detect about 200 photons in the OVII line during a $10^5$ s exposure if the metallicity of the gas is as large as observed in galaxy clusters. On average up to a few percent of the soft XRB could be resonantly scattered by this phase of the IGM and resonantly scattered photons should account for a significant fraction of the truly diffuse background at low energies. Close to bright X-ray sources like galaxy clusters or AGN the flux of scattered radiation will be further enhanced. Off-line blazars are the most promising illuminating sources. The scattered emission from AGN may also constrain the duration of the active phase of these objects. 
E.Churazov (MPA,IKI), M.Haehnelt (MPA,ITP), O.Kotov (IKI,MPA),   R.Sunyaev (MPA,IKI)