Ionization by galaxy cluster photons alters the ionization state of the nearby warm-hot intergalactic medium

The physical properties of the faint and extremely tenuous plasma in the far outskirts of galaxy clusters, the circumgalactic media of normal galaxies, and filaments of the cosmic web, remain one of the biggest unknowns in our story of large-scale structure evolution. Modeling the spectral features due to emission and absorption from this very diffuse plasma poses unique challenges, as both collisional and photo-ionisation processes must be accounted for. In this paper we study the ionisation by galaxy cluster photons in addition to the photo-ionisation by the cosmic UV/X-ray background and its impact on the ionisation balance. We model realistic spectra by taking into account the cosmic UV/X-ray background together with the emission from three different cool-core galaxy clusters: A262, A1795 and A2029, and illuminate the photo-ionised gas in the galaxy cluster vicinity. We assume the gas has temperatures between $10^{-3}$--$1$ keV ($\approx 10^4$--$10^7$ K), densities between $10^{-6}$--$10^{-1}$ cm$^{-3}$ and can be located between $r_{500}$ and $\sim 13$ Mpc from the cluster center. We find that depending on the distance from the galaxy cluster and the plasma properties, the total photo-ionisation rate can be higher than $10$% or even $100$% of the total ionization rate. We show how this affects the ionisation fractions of C IV, C V, C VI, N VII, O VI, O VII, O VIII, Ne VIII, Ne IX and Fe XVII ions and compare it with the ionization fractions with photo-ionization from cosmic background only as well as with plasma that is in collisional ionisation equilibrium. We assume a simplified model of a cosmic web filament and predict the column densities for different lines of sight.