Prospects of detecting soft X-ray emission from typical WHIM filaments around massive clusters and the coma cluster soft excess.

While hot ICM in galaxy clusters makes these objects powerful X-ray sources, the cluster's outskirts and overdense gaseous filaments might give rise to much fainter sub-keV emission. Cosmological simulations show a prominent 'focusing' effect of rich clusters on the space density of the warm-hot intergalactic medium (WHIM) filaments up to a distance of |$\sim 10\, {\rm Mpc}$| (∼ turnaround radius, rta) and beyond. Here, we use Magneticum simulations to characterize their properties in terms of integrated emission measure for a given temperature and overdensity cut and the level of contamination by the more dense gas. We suggest that the annuli |$(\sim 0.5-1)\times \, r_{ta}$| around massive clusters might be the most promising sites for the search of the gas with overdensity ≲ 50. We model spectral signatures of the WHIM in the X-ray band and identify two distinct regimes for the gas at temperatures below and above |$\sim 10^6\, {\rm K}$|⁠. Using this model, we estimate the sensitivity of X-ray telescopes to the WHIM spectral signatures. We found that the WHIM structures are within reach of future high spectral resolution missions, provided that the low-density gas is not extremely metal-poor. We then consider the Coma cluster observed by SRG/eROSITA during the CalPV phase as an example of a nearby massive object. We found that beyond the central r ∼ 40 arcmin (⁠|$\sim 1100\, {\rm kpc}$|⁠) circle, where calibration uncertainties preclude clean separation of the extremely bright cluster emission from a possible softer component, the conservative upper limits are about an order of magnitude larger than the levels expected from simulations. [ABSTRACT FROM AUTHOR]