Quasinormal modes of type II-perturbation for dilaton-Euler-Heisenberg black holes

Abstract: We study the quasinormal modes (QNMs) of Type II perturbations for dilaton-Euler-Heisenberg (dEH) black holes. These perturbations consist of coupled even-parity gravitational and dilaton perturbations together with odd-parity electromagnetic perturbations. The background is described by mass $M$, magnetic charge $Q_m$, and dilaton coupling difference $ζ=α-β$ to the Euler-Heisenberg term. To find the QNM frequencies, we need to find the parameter space of $(ζ, Q_m/M)$ that is free from vector ghosts. For the $\ell=1$ mode, the radiative block couples the dilaton and axial electromagnetic amplitudes, whereas the $\ell=2$ mode also contains metric amplitudes, so the frequencies are obtained from matrix-valued boundary-value problems. We calculate QNM frequencies for the fundamental ($n=0$) branch with direct integration and the Chebyshev pseudospectral method, and compare the two computations wherever their charge intervals overlap. We find that the $\ell=1,2$ modes in the $n=0$ branch remain damped over the ghost-free parameter domain, supporting the stability of dEH black holes against Type II perturbations. The Type I sector will be presented elsewhere; no claim is made here about that sector, higher multipoles, or overtones.