Marginally stable Schwarzschild-black-hole-non-minimally-coupled-Proca-field bound-state configurations

It has recently been revealed that, in curved black-hole spacetimes, non-minimally coupled massive Proca fields may be characterized by the existence of poles in their linearized perturbation equations and may therefore develop exponentially growing instabilities. Interestingly, recent numerical computations [H. W. Chiang, S. Garcia-Saenz, and A. Sang, arXiv:2504.04779] have provided compelling evidence that the onset of monopole instabilities in the composed black-hole-field system is controlled by the dimensionless physical parameter $\mu r_{-}$, where $\mu$ is the proper mass of the non-minimally coupled Proca field and $r_{-}\equiv (-2\alpha {)}^{1\mathrm{/}3}{r}_{\text{H}}$ is the radial location of the pole [here $\alpha$ is the non-minimal coupling parameter of the Einstein-Proca theory and $r_{\text{H}}$ is the radius of the black-hole horizon]. In the present paper we use {\it analytical} techniques in order to explore the physical properties of critical (marginally-stable) composed Schwarzschild-black-hole-nonminimally-coupled-monopole-Proca-field configurations. In particular, we derive a remarkably compact analytical formula for the discrete spectrum $\{\mu (r_{\text{H}},r_{-};n){\}}_{n=1}^{n=\mathrm{\infty }}$ of Proca field masses which characterize the critical black-hole-monopole-Proca-field configurations in the dimensionless regime $\frac{r_{-}-r_{\text{H}}}{r_{\text{H}}}\ll 1$ of near-horizon poles. The physical significance of the analytically derived resonance spectrum stems from the fact that the critical field mass ${\mu }_{\text{c}}\equiv \mu (r_{\text{H}},r_{-};n=1)$ marks the onset of instabilities in the Schwarzschild-black-hole-nonminimally-coupled-monopole-Proca-field system. In particular, composed black-hole-linearized-Proca-field configurations in the small-mass regime $\mu \le {\mu }_{\text{c}}$ of the Proca field are stable.