The role of the ergosphere in the Blandford-Znajek process
  The Blandford-Znajek process, one of the most promising model for powering the relativistic jets from black holes, was initially introduced as a mechanism in which the magnetic fields extract energy from a rotating black hole. We study the evolution of force-free electromagnetic fields on regular spacetimes with an ergosphere, which are generated by rapidly rotating stars. Our conclusive results confirm previous works, claiming that the Blandford-Znajek mechanism is not directly related to the horizon of the black hole. We also show that the radiated energy depends exponentially on the compactness of the star. 
Milton Ruiz, Carlos Palenzuela, Filippo Galeazzi and Carles Bona
gr-qc