Horizon Instability of Extremal Kerr Black Holes: Nonaxisymmetric Modes   and Enhanced Growth Rate
  We show that the horizon instability of the extremal Kerr black hole is associated with a singular branch point in the Green function at the superradiant bound frequency. We study generic initial data supported away from the horizon and find an enhanced growth rate due to nonaxisymmetric modes. The growth is controlled by the conformal weight of each mode. We speculate on connections to near-extremal black holes and holographic duality. 
Marc Casals, Samuel E. Gralla, and Peter Zimmerman
gr-qc hep-th