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 $\tau \equiv 1\mathrm{/}\mathrm{\Im }{\omega }_0$ of near-extremal Kerr-Newman black holes in the regime $Q\mathrm{/}{r}_{+}\le 0.9$ 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).