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arXiv · 2609.07326

Exact Cancellation of Horizon Modes in Kerr Ringdown

Kei-ichiro Kubota·Yusuke Manita·Hayato Motohashi·Daiki Watarai
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Abstract

Ringdown gravitational waves probe black hole spacetimes via quasinormal modes (QNMs). Recent studies have reported oscillatory features in QNM-filtered waveforms, and have led to conflicting interpretations as to whether they represent a near-horizon "direct wave". Reexamining a particle plunging into a Kerr black hole, we show that the saddle-point approximation underlying the proposed interpretation fails near the horizon. Our exact analysis reveals that every source-induced horizon-mode pole is canceled by an infinite tower of Matsubara zeros, independently of orbital and black-hole parameters. The first four stages of this sequential screening are also confirmed by time-domain numerical integration. Thus, within the near-horizon perturbative framework, source-induced horizon-mode signals do not survive at late times.

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Kei-ichiro Kubota, Yusuke Manita, Hayato Motohashi, Daiki Watarai. 2026-09-07. Exact Cancellation of Horizon Modes in Kerr Ringdown. https://arxiv.org/abs/2609.07326

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