GW231123: Extreme Black Hole Merger

• GW231123 is a gravitational-wave event showcasing the merger of two exceptionally massive, high-spin black holes situated in or above the pair‐instability mass gap.
• Its detection by Advanced LIGO, with robust Bayesian inference across multiple waveform models, constrains formation channels including hierarchical mergers, direct stellar collapse, and primordial origins.
• The event’s analysis informs astrophysical implications for intermediate-mass black hole assembly, gravitational lensing effects, and the role of waveform systematics in merger parameter estimation.

GW231123 denotes a gravitational-wave event detected during the fourth LIGO observing run, characterized by two exceptionally massive and rapidly spinning black holes. The primary lies at $m_1 = 137^{+22}_{-17}~M_\odot$ and the secondary at $m_2 = 103^{+20}_{-52}~M_\odot$, both with high spin parameters ($\chi_1 \simeq 0.9$, $\chi_2 \simeq 0.8$). The system’s configuration places it at or above the so-called pair-instability (PISN) mass gap, posing a direct challenge to standard predictions of stellar evolution. This event has catalyzed a broad array of theoretical investigations spanning stellar astrophysics, hierarchical mergers, primordial black holes, advanced waveform systematics, gravitational lensing, and instrumental noise systematics.

1. Detection, Signal Properties, and Parameter Inference

GW231123 was observed on 2023-11-23 by the Advanced LIGO Hanford and Livingston detectors, registering a network SNR of $\sim$22.5. The event’s inferred properties from Bayesian analysis over a suite of waveform models (NRSur7dq4, SEOBNRv5PHM, IMRPhenomTPHM, IMRPhenomXPHM, IMRPhenomXO4a) are:

• Primary mass: $137^{+22}_{-17}\,M_\odot$
• Secondary mass: $103^{+20}_{-52}\,M_\odot$
• Dimensionless spin magnitudes: $\chi_1 = 0.90^{+0.10}_{-0.19}$, $\chi_2 = 0.80^{+0.20}_{-0.51}$
• Luminosity distance: $0.7$--$m_2 = 103^{+20}_{-52}~M_\odot$0 Gpc ($m_2 = 103^{+20}_{-52}~M_\odot$1)
• Remnant mass: $m_2 = 103^{+20}_{-52}~M_\odot$2, with final spin $m_2 = 103^{+20}_{-52}~M_\odot$3

The event was exceptionally short ($m_2 = 103^{+20}_{-52}~M_\odot$40.1 s), exhibiting only a few observable cycles above the detector noise floor. Inference robustness against waveform systematics and Gaussian noise has been demonstrated, with the principal features—high component masses and spins—reproducible in zero-noise and noise-perturbed simulations, as well as by independent machine-learning reconstructions (Collaboration et al., 10 Jul 2025, Bini et al., 14 Jan 2026, Chatterjee et al., 11 Sep 2025).

Systematic uncertainties do remain considerable, with between-model discrepancies yielding posterior spreads of tens of solar masses and up to $m_2 = 103^{+20}_{-52}~M_\odot$5--$m_2 = 103^{+20}_{-52}~M_\odot$6 on spin parameters (Collaboration et al., 10 Jul 2025, Bini et al., 14 Jan 2026). However, the headline features persist across all current frameworks.

2. Physical Origin Scenarios: Hierarchical Mergers and Stellar Evolution

Standard stellar collapse models struggle to produce black holes with $m_2 = 103^{+20}_{-52}~M_\odot$7 due to PISN, which disrupts or mass-strips massive progenitors. As both GW231123 components sit in or above this mass gap, several formation mechanisms have been proposed:

Hierarchical Mergers. The prevailing scenario involves multiple generations of black-hole mergers in dense stellar clusters or active galactic nucleus (AGN) disks. Bayesian framework analyses, using population-informed mass and spin priors, overwhelmingly favor models where both GW231123 black holes originate from prior mergers (hierarchical generations $m_2 = 103^{+20}_{-52}~M_\odot$8), with $m_2 = 103^{+20}_{-52}~M_\odot$9 odds over alternative scenarios (Li et al., 10 Sep 2025, Li et al., 23 Jul 2025). Typical remnant spins from single mergers cluster at $\chi_1 \simeq 0.9$0; to recover the observed $\chi_1 \simeq 0.9$1, further hierarchical assembly or fine-tuned spin alignment is required (Passenger et al., 16 Oct 2025).

Stellar Physics. An alternative involves formation from a single, rapidly rotating massive star. Simulations of $\chi_1 \simeq 0.9$2 metal-poor helium cores, with realistic ranges of $\chi_1 \simeq 0.9$3 rates, show that stellar rotation can shift the PISN threshold upward to $\chi_1 \simeq 0.9$4--$\chi_1 \simeq 0.9$5. This enables the direct collapse of a massive, high-spin core into a black hole with $\chi_1 \simeq 0.9$6--$\chi_1 \simeq 0.9$7—sufficient to account for GW231123, but only with significant rotational support and suppressed angular-momentum transport (i.e., inefficient Spruit–Tayler dynamo) (Croon et al., 13 Aug 2025).

Population III Star Clusters and Isolated Pop III Binaries. High-resolution population-synthesis and direct N-body simulations indicate that GW231123-like systems can form via stellar or binary black hole mergers in Pop III clusters, with predicted rate densities $\chi_1 \simeq 0.9$8--$\chi_1 \simeq 0.9$9, matching observations. These channels require inefficient convective overshooting and a $\chi_2 \simeq 0.8$0 rate suppressed by $\chi_2 \simeq 0.8$1 relative to canonical values (Liu et al., 7 Oct 2025, Tanikawa et al., 2 Aug 2025).

Primordial Black Holes. If the GW231123 components are primordial (PBH), cosmological gas accretion is needed to both boost the masses and spin up low-spin seeds ($\chi_2 \simeq 0.8$2 at formation) to $\chi_2 \simeq 0.8$3. This scenario falls at the edge of X-ray and CMB constraints on PBH abundance (Luca et al., 13 Aug 2025).

Scenario	BH Masses	Spin Range	Key Astrophysical Feature
Hierarchical cluster/AGN	$\chi_2 \simeq 0.8$4	$\chi_2 \simeq 0.8$5--$\chi_2 \simeq 0.8$6	Requires high escape velocity, high merger retention
Direct single-star collapse	$\chi_2 \simeq 0.8$7 (rotating)	$\chi_2 \simeq 0.8$8--$\chi_2 \simeq 0.8$9	High core rotation, inefficient angular momentum transport
Primordial black holes	$\sim$0--$\sim$1 (after accretion)	up to $\sim$2	Cosmological accretion, tension with CMB limits

3. Lensing, Overlapping Signals, and Systematics

Given GW231123’s high inferred mass, analyses have explored the hypothesis that the event was gravitationally lensed and, consequently, its true source-frame mass is lower. Several lines of evidence are pertinent:

• Lensing Signatures: A comprehensive Bayesian analysis favouring a point-mass microlens embedded in external shear and convergence finds a log-Bayes factor of $\sim$3 (NRSur, embedded PL vs. unlensed), corresponding to $\sim$4 confidence for lensing (Goyal et al., 19 Dec 2025). Under this model, the source-frame total mass drops to $\sim$5–$\sim$6 and required component spins reduce to moderate values ($\sim$7, $\sim$8).
• Model-Independent Lensing Constraints: Residual-strain cross-correlation analyses constrain potential microlensing-induced amplitude modulations to $\sim$9 (95% C.L.), finding no strong evidence for lensing but emphasizing that current waveform systematics limit further progress (Chakraborty et al., 22 Dec 2025).
• Machine Learning and Overlapping Signal Hypothesis: Deep-learning frameworks confirm the detection and astrophysical origin of GW231123. Bayesian evidence also moderately favours double-signal models over a single-signal, suggesting the possibility of closely overlapping gravitational-wave events, which can mimic lensing phenomenology and mitigate between-model discrepancies (Hu et al., 19 Dec 2025, Chatterjee et al., 11 Sep 2025).
• Systematic Biases from Waveform Families and Glitches: Waveform choice introduces non-negligible systematic uncertainty in inferred masses and spins for GW231123; model-dependent offsets are larger than can be explained by Gaussian noise alone (Bini et al., 14 Jan 2026, Jan et al., 23 Dec 2025). Non-Gaussian noise transients, especially "microglitches", can artificially shift spin posteriors toward the maximal-spin boundary ($137^{+22}_{-17}\,M_\odot$0), a bias correctable by including explicit glitch models in parameter estimation (Ray et al., 8 Oct 2025).

4. Eccentricity, Spin, and Selection Effects

Standard waveform models lacking eccentricity corrections during merger and ringdown can misidentify the signature of residual eccentricity as high aligned spin. Simulations demonstrate that eccentric binaries with moderate spins (e.g., $137^{+22}_{-17}\,M_\odot$1, $137^{+22}_{-17}\,M_\odot$2) can be recovered under circular templates as systems with $137^{+22}_{-17}\,M_\odot$3 (Chandra et al., 13 Jun 2026). However, direct data fits to available high-eccentricity numerical-relativity catalog waveforms do not improve the likelihood relative to quasi-circular models—the data moderately disfavour eccentric configurations, and pure ringdown analyses yield remnant mass and spin values consistent with those from inspiral-merger-ringdown fits. For GW231123, full eccentric–precessing waveform analyses (TEOBResumS–Dalí) find no strong evidence for $137^{+22}_{-17}\,M_\odot$4 at 10 Hz; model selection modestly prefers quasicircular solutions (Jan et al., 23 Dec 2025, Chandra et al., 13 Jun 2026).

5. Exotic Physics and Robustness of BBH Interpretation

The binary black hole origin of GW231123 is supported overwhelmingly against cosmic-string burst alternatives, with Bayes factors exceeding $137^{+22}_{-17}\,M_\odot$5 (Cuceu et al., 28 Jul 2025). Tests for new gravitational-wave propagation physics (e.g., a nonzero graviton mass) demonstrate that unmodeled point-mass lensing can produce false signatures of propagation effects, resolving apparent anomalies in standard analyses once lensing is explicitly modeled (Wang et al., 9 Apr 2026).

The robustness of the BBH interpretation is further enhanced by:

• Low false-alarm rate and high SNR
• Consistent single-detector and network analyses
• Machine learning and model-agnostic reconstructions matching the predicted BBH spectrum

6. Astrophysical Implications and Future Prospects

GW231123 is a benchmark for the assembly channels of intermediate-mass black holes (IMBHs), the physics of the pair-instability mass gap, tests of general relativity in the highly nonlinear regime, and the environmental impact on gravitational-wave signals.

• Hierarchical Merger Chains: Most analyses favour a scenario in which both GW231123 black holes are $137^{+22}_{-17}\,M_\odot$62nd-generation merger remnants, originating from a sequence involving up to $137^{+22}_{-17}\,M_\odot$7 and $137^{+22}_{-17}\,M_\odot$8 first-generation BH progenitors, respectively, likely formed and retained in AGN disks or nuclear star clusters (Li et al., 23 Jul 2025, Li et al., 10 Sep 2025).
• Parameter Constraints on Stellar Evolution: The observation of GW231123 imposes constraints on Pop III stellar evolution, requiring both inefficient convective overshooting and a suppressed $137^{+22}_{-17}\,M_\odot$9 rate to enable direct collapse above the mass gap in the isolated binary channel (Tanikawa et al., 2 Aug 2025).
• Population Synthesis Predictions: Current and future observing runs are expected to record $103^{+20}_{-52}\,M_\odot$0 GW231123-like events if PBH or Pop III cluster scenarios are to explain the merger rate, enabling discrimination among hierarchical, PBH, and direct-collapse mechanisms through observed mass-spin correlations and redshift distributions (Luca et al., 13 Aug 2025, Liu et al., 7 Oct 2025).
• Gravitational Lensing Science: GW231123 demonstrates the confounding effects of lensing on gravitational-wave parameter estimation and the necessity of lensing-aware waveform models in both source inference and tests of fundamental physics (Goyal et al., 19 Dec 2025, Chan et al., 18 Dec 2025, Wang et al., 9 Apr 2026).

Continued improvements in waveform models (especially eccentric–precessing IMR families), data-driven noise modeling, machine learning for signal consistency, and deep-learning-accelerated lensing inference will be requisite for robust interpretation of the exceptional BBH mergers detected by current and next-generation gravitational-wave observatories.