Effects of Screw Instability on Extracting Energy from a Rotating Black Hole

Magnetic extraction of energy from a rotating black hole (BH) is discussed in the coexistence of the Blandford--Znajek (BZ) process and the magnetic coupling (MC) process by considering the effects of the screw instability of the magnetic field. It is shown that the screw instability affects the configuration of the magnetosphere and the evolution characteristics of the BH, augmenting the energy extracted in the BZ and MC processes. In addition, the observed energy and duration of several gamma-ray bursts can be fitted well by virtue of our model.