The structured pseudospectrum of Schwarzschild black hole

Abstract

The non-Hermitian nature of black hole quasinormal modes (QNMs) can give rise to remarkable spectrum instability. Such instability is naturally characterized by the pseudospectrum for the operator. However, conventional pseudospectrum analyses permit arbitrary operator perturbations that may not correspond to physically admissible modifications. In this work, we introduce the structured pseudospectrum into black hole perturbation theory. We construct two physically motivated perturbation spaces based on effective field theory and the Rezzolla--Zhidenko parametrization, and get their structured pseudospectra using two distinct norms. Our numerical results show that imposing physical constraints substantially reduces the accessible spectrum instability, especially for higher overtones. More importantly, at sufficiently small perturbation scales $\varepsilon$, the structured pseudospectrum around isolated QNMs exhibits approximately elliptical contours with preferred directions that depend on both the mode and the perturbation space, revealing an anisotropic structure beyond the familiar circular behavior of the conventional pseudospectrum.