Relation between the R₅₀₀, R_sig, extent, the integrated count rate within R₅₀₀, the integrated count rate within R_sig, and the flux inside R₅₀₀. Top-left, top-right, and bottom-left panels: the red dashed line labels the 1:1 relation, while bottom-right panel: relation of F₅₀₀ = CR_sig × 2 × 10⁻¹¹ (erg s⁻¹ cm⁻²)/(count s⁻¹). Top-left panel: the red dotted line shows the dividing line to get the very extended candidates discussed in Sect. 4.1.