Direct Waves in Black-Hole Binary Mergers: Insights from the Backwards One Body Model

TL;DR

This paper demonstrates that the Backwards One Body (BOB) model effectively captures the direct wave component in black-hole binary merger radiation, revealing its frequency behavior and improving understanding of merger-ringdown signals.

Contribution

The study shows how BOB models the non-QNM direct wave component using minimal parameters, providing new insights into its frequency characteristics and relation to the News frequency.

Findings

BOB accurately models the direct wave component in merger radiation.

The direct wave frequency is largely uncorrelated with the horizon frequency.

The direct wave tracks the News frequency at peak amplitude.

Abstract

The merger-ringdown radiation from a black hole binary merger is accurately modeled by a sum of linear quasinormal modes (QNMs). Recently, a non-QNM ``direct wave" component of the radiation, associated with prompt emission from a plunging perturber, has been identified. Motivated by the behavior of null geodesics perturbed from the remnant light ring, the Backwards One Body (BOB) approach has been shown to model the full merger-ringdown radiation to high accuracy, while using only a minimal number of parameters. In this work, using the P\"oschl--Teller potential, we first show how the BOB amplitude evolution can be recovered from the QNM pole contributions. We then apply rational filters to isolate the non-QNM content in BOB and numerical relativity waveforms. We show that BOB naturally captures the direct wave component of the merger radiation, explaining its accuracy near the waveform peak. Finally, we use BOB to show that the direct wave frequency is largely uncorrelated with the horizon frequency, even for high spin remnants, and instead tracks the News frequency at the time of the peak News amplitude.