Electric Charge and Magnetic Flux on Rotating Black Holes in a   Force-Free Magnetosphere
  The electric charge on rotating black holes is calculated to be ~ BJ in the force-free configuration of Ghosh (2000), with a horizon flux of ~ BM^2. This charge is gravitationally weak for B ~ 10^{15} G, so that the Kerr metric applies. Being similar to the electric charge of a magnetar, both electric charge and magnetic flux should be, in sign and order of magnitude, continuous during stellar collapse into a black hole. Extraction of the rotational energy from newly formed black holes may proceed by interaction with the magnetic field. Keywords:black hole physics --magnetic fields 
Hyun Kyu Lee (1,3), Chul H. Lee (1) and Maurice H.P.M. van Putten   (2,3) ((1)Department of Physics, Hanyang University, Seo...