On the universality of instability in the fundamental quasinormal modes of black holes

We elaborate on a criterion for the emergence of instability in the fundamental mode recently observed by Cheung {\it et al.}, as a universal phenomenon in the context of black hole perturbations. Such instability is characterized by an exponential spiral, deviating from the quasinormal frequencies due to an insignificant perturbation moving away from the compact object. Our analysis begins with a specific case involving a truncated P\"oschl-Teller potential for which we derive an explicit form of the criterion. Notably, it is shown analytically, contrary to other cases studied in the literature, that the fundamental mode is stable. These derivations are then generalized to a broader context, embracing two underlying mathematical formalisms. Specifically, the spiral is attributed to either the poles in the black hole's reflection amplitude or the zeros in the transmission amplitude. Additionally, we revisit and then generalize a toy model in which perturbations to the effective potential are disjointed, demonstrating that such a configuration invariably leads to instability in the fundamental mode, and the resulting outward spiral always occurs in the counter-clockwise direction. The derived criterion is not restricted to the fundamental mode but is generally relevant for the first few low-lying modes. We demonstrate numerically that the sprial's period and the frequency's relative deviation agree well with our analytical estimations.