2026A&A...707A.122G

Context. An excess of soft X-ray emission (∼0.2 - 1 keV) above the contribution from the hot intracluster medium (ICM) has been detected in a number of galaxy clusters, including the Coma cluster. The physical origin of this emitting medium above the hot ICM has not yet been determined, in particular, it is unclear whether it is thermal or nonthermal.
Aims. We investigate the gas phase and gas structure that reproduce the soft excess radiation from the cluster core to the outskirts best using simulations.
Method. By using the IllustrisTNG simulation (TNG300), we predict the radial profile of thermodynamic properties and the soft X-ray surface brightness of 138 clusters within 5 × r₂₀₀. Their X-ray emission was simulated for the hot ICM gas phase (T ≥ 10⁷ K), the entire warm-hot medium at a temperature T = 10^5 - 7^K (WARM), and for the diffuse and low-density warm-hot intergalactic medium (WHIM).
Results. The soft excess inside clusters appears to be produced by substructures of the WARM gas phase that host dense warm clumps, that is, the warm circumgalactic medium (WCGM), and the inner soft excess is strongly correlated with substructure and the WCGM mass fractions. Outside of the virial radius, the fraction of WHIM gas that is mostly inside filaments that are connected to clusters boosts the soft X-ray excess. The more diffuse the gas, the higher the soft X-ray excess beyond the virial region.
Conclusion. The thermal emission of the WARM gas phase in the form of WCGM clumps and WHIM diffuse filaments reproduces the soft excess emission that was observed up to the virial radius in Coma and in the inner regions of other massive clusters. Moreover, our analysis suggests that soft X-ray excess is a proxy of the dynamical cluster state and that higher excess is observed in the most unrelaxed clusters.