GW231123: a Binary Black Hole Merger with Total Mass 190-265 $M_{\odot}$

TL;DR

GW231123 presents one of the most massive binary-black-hole mergers observed to date, with a total mass in the 190–265 $M_\odot$ range and high spins on both components. A coherent LVK analysis across five precessing IMR waveform models yields a consistent picture of a high-mass system at $z\approx0.4$, including a remnant likely exceeding $200\,M_\odot$ from ringdown. Despite strong evidence for a black-hole merger, large waveform-systematics at high spins introduce substantial uncertainties in masses and orientation, and the data hint at at least one component lying in the pair-instability mass gap, suggesting non-standard formation channels such as hierarchical mergers or stellar-merger growth. The study discusses merger-rate implications, population-context checks, and several formation scenarios, while acknowledging alternative interpretations (eccentricity, lensing, or exotic sources) that require further investigation as more massive events are observed.

Abstract

On 2023 November 23 the two LIGO observatories both detected GW231123, a gravitational-wave signal consistent with the merger of two black holes with masses $137^{+23}_{-18}\, M_\odot$ and $101^{+22}_{-50}\, M_\odot$ (90\% credible intervals), at luminosity distance 0.7-4.1 Gpc and redshift of $0.40^{+0.27}_{-0.25}$, and a network signal-to-noise ratio of $\sim$20.7. Both black holes exhibit high spins, $0.9^{+0.10}_{-0.19}$ and $0.80^{+0.20}_{-0.52}$ respectively. A massive black hole remnant is supported by an independent ringdown analysis. Some properties of GW231123 are subject to large systematic uncertainties, as indicated by differences in inferred parameters between signal models. The primary black hole lies within or above the theorized mass gap where black holes between 60-130 $M_\odot$ should be rare due to pair instability mechanisms, while the secondary spans the gap. The observation of GW231123 therefore suggests the formation of black holes from channels beyond standard stellar collapse, and that intermediate-mass black holes of mass $\sim$200 $M_\odot$ form through gravitational-wave driven mergers.

Figures (6)

• Figure 1: The gw event GW231123 as observed by the LIGO Hanford (left panels) and LIGO Livingston (right panels) detectors. Time is measured relative to 2023 November 23 at 13:54:30.619 UTC. The top panels show the time-domain strain data (black), sampled at 1024 Hz, whitened and then bandpass-filtered with a passband from 20 Hz to 256 Hz LIGOScientific:2019hgc. Also shown are the point-estimate whitened waveform from the cWB-BBH search (red), the 90% credible interval of whitened waveforms inferred from a coherent Bayesian analysis using the combined samples from five bbh waveform models (blue bands), and the 90% credible interval inferred from BayesWave using a generic wavelet-based model (shaded purple). The vertical axis is in units of the noise standard deviation, $\sigma_{\rm noise}$. The bottom panels display the corresponding whitened time-frequency representations of the strain data, obtained using a continuous wavelet transform (CWT) with a Morlet–Gabor wavelet. The color scale is in units of the amplitude of the CWT coefficients.
• Figure 2: Mismatch accuracy of the waveform models considered in this paper against 1123 NR simulations at a total mass of 300$M_\odot$ and a range of inclinations between $\iota=0$ and $\pi/2$. The vertical dashed line at a mismatch of 0.0028 shows the conservative criterion discussed in the text.
• Figure 3: The posterior distribution of the primary and secondary source masses. We show the posterior distribution resulting from equally combining samples from five waveform models that include precession and higher-order multipoles (purple). We separately show the posterior distribution obtained with NRSur (green dash dot). We compare against estimates for the source frame masses of GW190521 LIGOScientific:2020iuhLIGOScientific:2020ufjLIGOScientific:2021vkt. Each contour, as well as the colored horizontal and vertical lines, shows the 90% credible intervals. In blue dashed we show the posterior predictive distribution for the largest BH mass $m_\mathrm{max}^\mathrm{obs}$ in mock catalogs similar to GWTC-3 LIGOScientific:2021vktKAGRA:2021duu; see Section \ref{['sec:astro']}. The solid orange bands show the putative mass gap from (pulsational) pair instability from 60--130 $M_{\odot}$.
• Figure 4: The posterior distribution of the primary and secondary spin magnitudes. We show the posterior distribution based on the combined samples (purple) and from the NRSur7dq4 waveform model (NRSur, green dash dot). Each contour, as well as the colored horizontal and vertical lines, shows the 90% credible intervals.
• Figure 5: Two-dimensional frequency and damping time posterior distribution ($90\%$ credible levels), when starting the analysis at late times and assuming a single damped sinusoid with positive frequency; the combined IMR estimates for the longest-lived ${\ell m n} = {220}$ QNM are shown for comparison. For visualisation purposes, we display up to $\tau_1 = 45$ms, while the posterior tail extends up to $\tau_1 = 80$ms.