Force-free magnetosphere attractors for near-horizon extreme and near-extreme limits of Kerr black hole

Abstract

We propose a new approach to find magnetically-dominated force-free (FF) magnetospheres around highly spinning black holes, relevant for models of astrophysical jets. Employing the near-horizon extreme Kerr (NHEK) limit of the Kerr black hole, any stationary, axisymmetric and regular FF magnetosphere reduces to the same attractor solution in the NHEK limit with null electromagnetic field strength. We use this attractor solution as the universal starting point for perturbing away from the NHEK region in the extreme Kerr spacetime. We demonstrate that by going to second order in perturbation theory, it is possible to find magnetically dominated magnetospheres around the extreme Kerr black hole. Furthermore, we consider the near-horizon near-extreme Kerr (near-NHEK) limit that provides access to a different regime of highly spinning black holes. Also in this case we find a novel FF attractor, which can be used as the universal starting point for a perturbative construction of FF magnetospheres. Finally, we discuss the relation between the NHEK and near-NHEK attractors.