Emission measure EM_τ of shocked gas with the ionization parameter τ below a given threshold, relative to the emission measure EM_cl of unshocked gas at the same distance from the cluster center (see Eq. 7). The solid and dashed purple lines show the ratio EM_τ/EM_cl for τ = 10¹¹ and 10¹² cm⁻³ s, respectively. For the shock velocity, we use the velocity v_r of a point mass on a parabolic orbit. The blue and red lines show the overdensity of baryons as a function of radius and the product of overdensity and radius, respectively. The reciprocal of the latter quantity is the key quantity that determines the ratio EM_τ/EM_cl. The plots show that regions with τ ∼ 10¹¹ cm⁻³ s can contribute ∼10% to the line-of-sight emission measure, while for τ ∼ 10¹² cm⁻³ s the contributions from shocked and unshocked regions can be comparable. This conclusion holds for essentially any projected distance from the cluster center, although projection effects may also play a role.