Numerical evidence for universality in the relaxation dynamics of near-extremal Kerr-Newman black holes

The coupled gravitational-electromagnetic quasinormal resonances of charged rotating Kerr-Newman black holes are explored. In particular, using the recently published numerical data of Dias, Godazgar, and Santos [Phys. Rev. Lett. 114, 151101 (2015)], we show that the characteristic relaxation times $\tau\equiv 1/\Im\omega_0$ of near-extremal Kerr-Newman black holes in the regime $Q/r_+\leq 0.9$ are described, to a very good degree of accuracy, by the simple universal relation $\tau\times T_{\text{BH}}=\pi^{-1}$ (here $Q, r_+$, and $T_{\text{BH}}$ are respectively the electric charge, horizon radius, and temperature of the Kerr-Newman black hole, and $\omega_0$ is the fundamental quasinormal resonance of the perturbed black-hole spacetime).