Pseudospectrum of Reissner-Nordström black holes: Quasinormal mode instability and universality

Black hole spectroscopy is a powerful tool to probe the Kerr nature of astrophysical compact objects and their environment. The observation multiple ringdown modes in gravitational waveforms could soon lead high-precision spectroscopy, so it critical understand if quasinormal mode spectrum stable against perturbations. It was recently shown that pseudospectrum can shed light on spectral stability black modes. We study Reissner-Nordstr\"om spacetimes we find instability scalar gravitoelectric subextremal extremal holes, extending similar findings for Schwarzschild spacetime. asymptotic structure pseudospectral contour levels same By making different gauge choices hyperboloidal slicing spacetime, broad features pseudospectra are remarkably gauge-independent. gravitational-led electromagnetic-led holes exhibit "strong" isospectrality: not only coincides, but whole both classes observe conformal duality between horizon spacetime boundaries at infinity responsible such isospectrality property.