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

Abstract

We use numerical relativity to study the merger and ringdown stages of "superkick" binary black hole systems (those with equal mass and antiparallel 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 M ⊙ . For massive binary black hole (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 full signal.

On the ease of excitation of black hole ringing: Quantifying the importance of overtones by the excitation factors

“…At j = 0.7 which is close to the spin parameter of the remnant black hole of GW150914, we found that the first two highest values of |E 22n | are at n = 4 and 5, which is consistent with the data analysis of GW150914-like numerical relativity waveform SXS:BBH:0305 in Ref. [30] and the recent fitting analysis that has investigated the universality of the importance of overtones [51]. Utilizing the result in Ref.…”

“…This surprising result was followed by other extensive studies [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. Also, it was recently confirmed [51] that the hierarchy of the mode amplitudes and the significance of the 4th and 5th overtones are insensitive to the initial condition of superkick configuration of BBHs (those with equal mass and anti-parallel spins). In spite of the supporting evidence indicating the importance of overtones, it is still an open question why the overtones are highly excited at the early ringdown.…”

“…In the future work, we will study the issue of to what extent the condition of(38) holds for the ringdown signals sourced by BBH mergers. In Ref [51],. it was found that the hierarchy and absolute values of the excitation coefficients of ringdown signals of the superkick configuration of BBH mergers (those with equal mass and anti-parallel spins) are insensitive to initial conditions.…”

Angular emission patterns of remnant black holes

“…Retrograde excitations have been studied in the case of extreme mass ratio inspirals [29][30][31][32] using black hole perturbation theory [7]. Features of the ringdown waveforms have also been numerically studied in superkick BBH systems with equal mass [93], while not many studies have been done in the intermediate mass ratio inspirals [94,95].…”

Evidence of large recoil velocity from a black hole merger signal

“…In particular, because the kick is mostly imparted near the merger [2], the Doppler shift only affects the merger and ringdown part of the signal. This can lead to biases in the measurement of the remnant mass m f [21,45], Each purple marker indicates a kick posterior sample; an arrow drawn from the origin to the marker would show the kick vector v f . The outer radius of the sphere corresponds to v f = 2500 km/s.…”