Isospectrality breaking in the axial quasinormal modes of Reissner-Nordström black holes in dynamical Chern-Simons gravity

Abstract

We study odd-parity perturbations of Reissner-Nordström black holes in dynamical Chern-Simons gravity. The dCS action is parity invariant because $\vartheta\,{}^{*}RR$ is the product of two pseudoscalars; nevertheless, the pseudoscalar perturbation has odd parity and enters only the axial sector at linear order and so lifts the axial--polar isospectrality of general relativity. We derive the finite-charge couplings between the pseudoscalar and the axial gravitational and electromagnetic perturbations directly from the linearized field equations. Both directions are governed by the radial kernel $(Mr-Q^2)Δ/r^8$, and at nonzero charge the pseudoscalar couples to $Ψ+χΨ_e$, with $χ=2Q/[r\sqrt{(l-1)(l+2)}]$. After diagonalizing the Einstein--Maxwell sector, we compute branch-resolved quasinormal-mode shifts with exterior complex scaling for the photon-sphere modes and Frobenius shooting for the near-extremal zero-damping modes. For the Schwarzschild $l=2$ fundamental mode we obtain $β\,δω=-0.7733-0.3946\,i$ (units $M=α=1$). Off resonance, the gravitational-led shift scales as $ζ\,l^3$ at $Q=0$ but as $ζ\,l^2/Q$ at fixed nonzero charge and large $l$. The electromagnetic-led branch becomes exactly degenerate with the pseudoscalar mode at $Q_c=2M\sqrt{l(l+1)+1}/[l(l+1)+2]$, where the pseudoscalar potential coincides with the polar partner of the electromagnetic-led potential; there the breaking is $\mathcal{O}(\sqrtζ)$, $M|Δω|\simeq2.4\sqrtζ$ for $l=2$, and the shift reverses sign across $Q_c$. Near extremality the photon-sphere shifts remain finite, whereas the non-degenerate fundamental gravitational-led zero-damping mode is suppressed, $δω/ω\simeq6.0\times10^{3}\,ζ\,(1-Q^2/M^2)$ for $l=2$.