Superradiant instability of a charged regular black hole

Abstract

We show that a charged, massive scalar field in the vicinity of an electrically-charged Ay\'on-Beato-Garc\'ia (ABG) regular black hole has a spectrum of quasibound states that (in a certain parameter regime) grow exponentially with time, due to black hole superradiance. Superradiant quasibound states are made possible by the enhancement of the electrostatic potential at the horizon in nonlinear electrodynamics; in contrast, the Reissner-Nordstr\"om black hole does not possess such superradiant quasibound states. Here we compute the spectrum for a range of multipoles $\ell$ across the parameter space, and we find the fastest growth rate in the monopole mode. We find that a regular black hole with a small charge can still trigger a significant superradiant instability if the charge-to-mass ratio of the field is compensatingly large. We estimate the amount of black hole mass that can be deposited in the scalar field, finding an upper bound of circa $20\%$ in the extreme charge scenario. Finally, we consider the stationary bound states at the superradiant threshold, and we conjecture that, due to this instability, the ABG black hole will evolve towards a configuration with charged scalar hair.