(a) GRB prompt emission energy, E_γ,beam vs. the jet-powering efficiency, κ is shown. The possible parameter space of κ is represented by the limits of the error bar, the mean of these limits are marked by the solid black circles. The maximum efficiency of the BZ mechanism is marked by the red dashed line. (b) The estimated parameter space of the mass of the black hole central engines of the eight long GRBs listed in the Table 1 are represented by the limits of the error bars, the mean of these limits are marked by the dark red diamonds. For comparison, masses of the black holes observed by LIGO in binary black hole mergers (Abbott et al. 2019), and the masses of the galactic black holes estimated in X-ray binaries (Wiktorowicz et al. 2014), are shown in green circles and blue squares, respectively. The secondary merger component of unknown nature detected by LIGO in GW190814 is shown in the purple circle (Abbott et al. 2020).