  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).
