Zero modes and oscillatory instabilities of a Lorentz-violating Kalb-Ramond field on a Schwarzschild background

Abstract

We study equilibrium configurations and linear perturbations of a Lorentz-violating Kalb--Ramond field with a quartic symmetry-breaking potential and a nonminimal Riemann coupling on a fixed Schwarzschild background. For static spherical configurations, the electric component is determined algebraically by a characteristic function that can develop a finite-radius double root. Approaching this degenerate configuration, the monopole electric response scales as $|\widehat e(ω,r_c)|\propto(γ_c-γ)^{-1/2}$, while the propagating monopole amplitude remains regular, showing that the enhancement originates from the algebraic constraint rather than from a dynamical instability. For higher multipoles, we obtain an exact tower of zero-frequency modes, $ξ_{\ell n}=-(\ell+n+1)(\ell+n+2)/3$. For $\ell=1$, the finite-frequency spectrum contains two distinct low-frequency branches. As the Riemann coupling becomes more negative, the corresponding purely imaginary unstable modes coalesce and leave the imaginary axis as $ω_\pm=\pmω_R+iω_I$, producing an oscillatory instability. Near the merger, the real-part splitting follows a square-root law.