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Keywords:

  • accretion, accretion discs;
  • black hole physics;
  • hydrodynamics;
  • shock waves;
  • galaxies: active

ABSTRACT

The Blandford–Znajek (BZ) mechanism has usually been studied in the literature for accretion with considerably high angular momentum leading to the formation of either a cold Keplerian disc or a hot and geometrically thick sub-Keplerian flow as described within the framework of advection-dominated accretion flow/radiatively inefficient accretion flow. However, in nearby elliptical galaxies, as well as for our own Galactic Centre, accretion with very low angular momentum is prevalent. Such quasi-spherical strongly sub-Keplerian accretion has complex dynamical features and can accommodate stationary shocks. In this Letter, we present our calculation for the maximum efficiency obtainable through the BZ mechanism for complete general relativistic weakly rotating axisymmetric flow in the Kerr metric. Both shocked and shock-free flows have been studied in detail for rotating and counter-rotating accretion. Such a study has never been done in the literature before. We find that the energy extraction efficiency is low, about 0.1 per cent, and increases by a factor of 15 if the ram pressure is included. Such an efficiency is still much higher than the radiative efficiency of such optically thin flows. For the BZ mechanism, shocked flow produces a higher efficiency than the shock-free solutions and retrograde flow provides a slightly larger value of the efficiency than that for the prograde flow.