  The nature of the gamma-ray burst (GRB) central engine still remains an
enigma. Entities widely believed to be capable of powering the extreme jets are
magnetars and black holes. The maximum rotational energy that is available in a
millisecond magnetar to form a jet is ~10^52 erg. We identify 8 long GRBs whose
jet opening angle corrected energetics of the prompt emission episode are
>10^52 erg with high confidence level and therefore, their central engines are
expected to be black holes. Majority of these GRBs present significant emission
in sub-GeV energy range. The X-ray afterglow light curves of these bursts do
not show any shallow decay behaviour such as a plateau, however, a few cases
exhibit flares and multiple breaks instead of a single power-law decay. For a
minimum mass of the black hole (~2 Msun), we find the efficiency of producing a
jet from its rotational energy to range between 2%-270%. Highly energetic jets
requiring high efficiencies implies that either the mass of these black holes
are much larger or there are, in addition, other sources of energy which power
the jet. By considering the Blandford-Znajek mechanism of jet formation, we
estimate the masses of these black holes to range between ~2-60 Msun. Some of
the lighter black holes formed in these catastrophic events are likely
candidates to lie in the mass gap region (2-5 Msun).
