Quasinormal Modes of a Rotating Hořava Black Hole: Two Perturbative Approaches to Nonseparability

Abstract

Quasinormal modes (QNMs) provide a powerful probe of strong-field gravity. We study fundamental scalar quasinormal modes of an asymptotically flat rotating black hole in the low-energy sector of Hořava gravity, whose exact scalar equation is nonseparable due to rotation-induced Lorentz violation. We develop two complementary approximations that both preserve the exact Kerr sector: Method I applies a slow-rotation approximation only to the nonseparable Lorentz-violating part, whereas Method II employs a small-$η$ biorthogonal spheroidal projection, with $η$ the Lorentz-violating parameter. Using residual-controlled continuation and symmetric finite differences, validated by a Kerr continued-fraction benchmark and an independent radial spectral method, we compute the linear frequency response. Axisymmetric modes provide an exact Kerr null test. For the corotating modes studied, the real-frequency response is positive and increases with spin. At $a/M=0.9$, the two methods reveal a roughly $2\%$ angular correction to the real response, while the imaginary response is nonmonotonic and highly sensitive to angular projection due to internal cancellation. These results highlight the distinct rotational signatures of Lorentz violation in black-hole ringdown.