Resonant scattering of X-rays by the warm intergalactic medium

Abstract

For the low-density filamentary and sheet-like structures in the warm (∼104 to ∼106 K) intergalactic medium (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. As a result of 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 ∼1–2 eV) may contain up to 50 per cent of the total 0.5–1 keV emission of the filament. For a nearby (z≲0.1) filament with a Thomson optical depth of 10−4, XMM should detect about 200 photons in the O vii line during a 105-s exposure if the metallicity of the gas is as large as observed in galaxy clusters. On average up to a few per cent 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 active galactic nuclei (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.