Mean surface brightness vs. core radius. The mean surface brightness of about one quarter of the clusters with r_c > 100 kpc is below 43.35 erg s⁻¹ Mpc⁻² (blue line) in the [0.5–2] keV band within a radius of 300 kpc. The vertical dotted line marks the minimum core radius considered for computing the fraction of low surface brightness clusters. Three clusters (observed by two telescopes) appear twice in the figure, but were counted only once in the accounting of the low surface brightness fraction. The individual values can be found in Table A.1. The points are color-coded by μ₃₀₀ to facilitate the comparison with Fig. 7.