Universal features of gravitational waves emitted by superkick binary black hole systems

TL;DR

This paper demonstrates universal features of gravitational waves from superkick binary black hole mergers, showing that post-peak waveforms are largely parameter-insensitive and can be modeled effectively, with implications for improved parameter estimation.

Contribution

It introduces a universal description of gravitational wave evolution in superkick black hole mergers and evaluates the effectiveness of the BOB model and ringdown analysis for parameter estimation.

Findings

Waveforms normalized by peak values are insensitive to progenitor spins.

The BOB model accurately describes the mass quadrupole wave evolution.

Ringdown signals significantly improve parameter estimation for massive black hole binaries.

Abstract

We use numerical relativity to study the merger and ringdown stages of "superkick" binary black hole systems (those with equal mass and anti-parallel spins). We find a universal way to describe the mass and current quadrupole gravitational waves emitted by these systems during the merger and ringdown stage: (i) The time evolutions of these waves are insensitive to the progenitor's parameters (spins) after being normalized by their own peak values. (ii) The peak values, which encode all the spin information of the progenitor, can be consistently fitted to formulas inspired by post-Newtonian theory. We find that the universal evolution of the mass quadrupole wave can be accurately modeled by the so-called Backwards One-Body (BOB) model. However, the BOB model, in its present form, leads to a lower waveform match and a significant parameter-estimation bias for the current quadrupole wave. We also decompose the ringdown signal into seven overtones, and study the dependence of mode amplitudes on the progenitor's parameters. Such dependence is found to be insensitive to the overtone index (up to a scaling factor). Finally, we use the Fisher matrix technique to investigate how the ringdown waveform can be at least as important for parameter estimation as the inspiral stage. Assuming the Cosmic Explorer, we find the contribution of ringdown portion dominates as the total mass exceeds ~ 250 solar mass. For massive BBH systems, the accuracy of parameter measurement is improved by incorporating the information of ringdown -- the ringdown sector gives rise to a different parameter correlation from inspiral stage, hence the overall parameter correlation is reduced in the full signal.