F_γ in the 100 MeV−100 GeV band vs. F_X in the 0.5−10 keV band, from the Fermi 3FGL catalog, for all OUTs in our sample. The symbol coding is the same as in Fig. 10. The F_γ thresholds above which 100%, 98%, and 96% of the BZB sample would be detected in X-rays when their exposure times have been scaled down to 5 ks, are plotted in solid, dashed and dotted black lines, respectively.