Superradiant instability of magnetic black holes

Black hole superradiance has proven to be very valuable in several realms of gravitational physics and holds a promising discovery potential. In this paper, we consider the superradiant instability of magnetically charged, rotating black holes and find a number of important differences with respect to neutral ones. Considering massive charged bosonic fields, we find that the instability timescale is much shorter, and this is true even if the black hole contains an order-one number of magnetic monopoles, or merely a single one, and possesses either low, moderate, or large values of angular momentum. In particular, the instability is drastically faster than the radiative-decay time of charged pions, potentially making it physically relevant. Furthermore, our analysis identifies the most unstable modes as a class of monopole spheroidal harmonics, which we dub north and south monopole modes, whose morphology is markedly different from the ones in standard superradiance since they extend along the rotational axis. For completeness, we also study the quasinormal mode spectrum and amplification factors of charged massless fields, finding no evidence of instabilities in that case.