Impact of the cosmic UV background photoionization on the X-ray detectable phases. Top: Photon luminosity per unit of emission measure in the 0.5–2 keV band of the photoionized WHIM at different density (x-axis) and temperature (y-axis) by assuming Z = 0.2Z_⊙. Towards the high density (low ionization parameter) end, the radiative cooling reaches the CIE limit and is simply a function of temperature. When Δ_b ≲ 1, the radiative cooling keeps high at low temperature. Bottom: Same as the right panel of Fig. 10 but calculated using the photoionization model. By taking the cosmic UV background photoionization into account, the size of the 50% enclosed phases contour is marginally larger than that of the CIE condition.