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The physical properties of the faint and extremely tenuous plasma in the faroutskirts of galaxy clusters, the circumgalactic media of normal galaxies, andfilaments of the cosmic web, remain one of the biggest unknowns in our story oflarge-scale structure evolution. Modelling the spectral features due toemission and absorption from this very diffuse plasma poses a challenge, asboth collisional and photo-ionisation processes must be accounted for. In thispaper, we study the ionisation by photons emitted by the intra-cluster mediumin addition to the photo-ionisation by the cosmic UV/X-ray background on gas inthe vicinity of galaxy clusters. For near massive clusters such as A2029, theionisation parameter can no longer describe the ionisation balance uniquely.The ionisation fractions (in particular of C IV, C V, C VI, N VII, O VI, O VII,O VIII, Ne VIII, Ne IX, and Fe XVII) obtained by taking into account thephotoionisation by the cosmic background are either an upper or lower limit tothe ionisation fraction calculated as a function of distance from the emissionfrom the cluster. Using a toy model of a cosmic web filament, we predict howthe cluster illumination changes the column densities for two differentorientations of the line of sight. For lines of sight passing close to thecluster outskirts, O VI can be suppressed by a factor of up to $4.5$, O VII bya factor of $2.2$, C V by a factor of $3$, and Ne VIII can be boosted by afactor of $2$, for low density gas.

Galaxy cluster photons alter the ionisation state of the nearby warm-hot intergalactic medium