GW231028: Massive BBH Merger & Ringdown Overtone

• The paper reveals that GW231028 is a high-mass binary black hole merger with strongly positive effective spin and a remnant mass bordering the intermediate-mass regime.
• It employs combined Bayesian parameter estimation across four waveform models to address systematic variations and asymmetries in mass and spin.
• The dedicated ringdown analysis decisively detects the first overtone mode, providing a robust no-hair test that is consistent with general relativity.

GW231028_153006, often abbreviated GW231028, is a binary black hole (BBH) coalescence observed during the first part of the fourth LIGO–Virgo–KAGRA observing run (O4a) and included in GWTC-4.0 as a high-purity, validated event with $p_{\rm astro} \geq 0.5$ and false alarm rate $< 1\,\mathrm{yr}^{-1}$ (Collaboration et al., 25 Aug 2025). Within the O4a sample it is notable for its high source-frame total mass, strongly positive effective aligned spin, and remnant in the intermediate-mass black hole regime as inferred from catalog parameter estimation (Collaboration et al., 25 Aug 2025). It is also the subject of a dedicated ringdown analysis that reports decisive evidence for the first overtone mode $(\ell,m,n)=(2,2,1)$ beginning at $10\,M$ after the signal peak, together with a no-hair test consistent with general relativity (Wang, 10 Sep 2025).

1. Detection and catalog placement

GW231028_153006 has UTC coalescence time 2023-10-28 15:30:06, as encoded in the event name (Collaboration et al., 25 Aug 2025). In the O4a offline analyses, the event was observed with LIGO Hanford (H) and LIGO Livingston (L); Virgo (V) and KAGRA (K) were not used (Collaboration et al., 25 Aug 2025). It passed validation, is included in the parameter-estimation set, and is classified as a BBH (Collaboration et al., 25 Aug 2025).

Pipeline	FAR	Network SNR
CWB-BBH	$0.0012\,\mathrm{yr}^{-1}$	$22.4$
GstLAL	$< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$	$21.0$
MBTA	$< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$	$21.9$
PyCBC	$< 1\,\mathrm{yr}^{-1}$0	$< 1\,\mathrm{yr}^{-1}$1

All four pipelines report $< 1\,\mathrm{yr}^{-1}$2 (Collaboration et al., 25 Aug 2025). Representative per-detector signal-to-noise ratios vary slightly across pipelines: PyCBC gives $< 1\,\mathrm{yr}^{-1}$3, GstLAL gives $< 1\,\mathrm{yr}^{-1}$4, MBTA gives $< 1\,\mathrm{yr}^{-1}$5, and CWB-BBH gives $< 1\,\mathrm{yr}^{-1}$6, with the network value computed for two detectors as $< 1\,\mathrm{yr}^{-1}$7 (Collaboration et al., 25 Aug 2025).

The catalog places GW231028_153006 among the O4a compact binary coalescence candidates identified by at least one search algorithm with $< 1\,\mathrm{yr}^{-1}$8 and not vetoed during event validation (Collaboration et al., 25 Aug 2025). Its significance and inclusion in the detailed parameter-estimation sample make it part of the subset used for source-population and strong-field analyses rather than merely an alert-level candidate (Collaboration et al., 25 Aug 2025).

2. Source properties

Under the default LVK “Planck 2015” cosmology, the source-frame total mass is reported as $< 1\,\mathrm{yr}^{-1}$9, with mass ratio $(\ell,m,n)=(2,2,1)$0 and chirp mass $(\ell,m,n)=(2,2,1)$1 (Collaboration et al., 25 Aug 2025). The catalog uses the standard relations

$(\ell,m,n)=(2,2,1)$2

and notes that $(\ell,m,n)=(2,2,1)$3 with an approximate $(\ell,m,n)=(2,2,1)$4 range $(\ell,m,n)=(2,2,1)$5–$(\ell,m,n)=(2,2,1)$6, derived from the quoted $(\ell,m,n)=(2,2,1)$7 interval (Collaboration et al., 25 Aug 2025).

Spin inference indicates a distinctly positive aligned component. The effective aligned spin is

$(\ell,m,n)=(2,2,1)$8

while the combined “MIXED” posterior gives $(\ell,m,n)=(2,2,1)$9 for the effective precession parameter (Collaboration et al., 25 Aug 2025). The primary spin is likely large, with $10\,M$0 with $10\,M$1 probability, although tilt angles are not tightly constrained and the posterior remains broadly consistent with a net aligned-spin configuration (Collaboration et al., 25 Aug 2025).

The luminosity distance is approximately $10\,M$2 and the redshift is approximately $10\,M$3 (Collaboration et al., 25 Aug 2025). The catalog emphasizes the standard two-detector degeneracy between $10\,M$4 and inclination, so inclination is not tightly constrained (Collaboration et al., 25 Aug 2025). Likewise, no per-event sky area is quoted in the catalog summary; with only H and L observing in O4a, $10\,M$5 localizations are generally $10\,M$6–$10\,M$7 (Collaboration et al., 25 Aug 2025).

The event exhibits broad and model-dependent component-mass posteriors. The combined posterior implies medians near $10\,M$8 and $10\,M$9, but the credible ranges are broad and model-dependent, and the catalog does not highlight single-model medians for $0.0012\,\mathrm{yr}^{-1}$0 and $0.0012\,\mathrm{yr}^{-1}$1 because of systematic variations and multimodality (Collaboration et al., 25 Aug 2025).

3. Parameter estimation and modeling systematics

Bayesian parameter estimation for GW231028_153006 was performed with bilby plus dynesty, using agnostic priors: uniform in redshifted component masses, uniform in spin magnitudes, isotropic spin orientations and binary orientation, isotropic sky location, uniform in merger time and phase, and a distance prior corresponding to a uniform rate in comoving volume and time (Collaboration et al., 25 Aug 2025). Detector-frame and source-frame masses are related by

$0.0012\,\mathrm{yr}^{-1}$2

with redshift conversion determined by the adopted Planck 2015 cosmology (Collaboration et al., 25 Aug 2025).

For BBHs, including GW231028_153006, the catalog combines equal numbers of posterior samples from four waveform families: IMRPhenomXPHM_SpinTaylor, SEOBNRv5PHM, NRSUR7DQ4, and IMRPhenomXO4a (Collaboration et al., 25 Aug 2025). The reported “MIXED” results are intended to mitigate model systematics, which are particularly relevant for this event (Collaboration et al., 25 Aug 2025). Both higher-order modes and precession are treated by the default BBH models, and $0.0012\,\mathrm{yr}^{-1}$3 is explicitly inferred (Collaboration et al., 25 Aug 2025).

A salient feature of GW231028_153006 is the presence of significant systematic variations across waveform models for masses and spins (Collaboration et al., 25 Aug 2025). IMRPhenomXO4a shows a secondary mode at high $0.0012\,\mathrm{yr}^{-1}$4, and none of the four models closely agree on the exact component masses (Collaboration et al., 25 Aug 2025). This makes the combined “MIXED” posterior especially important for any catalog-level use of the event.

No data-quality issues requiring mitigation were identified for GW231028_153006 (Collaboration et al., 25 Aug 2025). Cross-checks with minimally modeled reconstructions show no evidence for missing waveform content beyond standard compact-binary-coalescence physics: BAYESWAVE gives on-source overlap $0.0012\,\mathrm{yr}^{-1}$5, off-source $0.0012\,\mathrm{yr}^{-1}$6, $0.0012\,\mathrm{yr}^{-1}$7; CWB-2G gives on-source $0.0012\,\mathrm{yr}^{-1}$8, off-source $0.0012\,\mathrm{yr}^{-1}$9, $22.4$0; and CWB-BBH gives on-source $22.4$1, off-source $22.4$2, $22.4$3 (Collaboration et al., 25 Aug 2025).

4. Remnant properties and astrophysical significance

The catalog reports a remnant black hole mass of approximately $22.4$4 and final dimensionless spin $22.4$5 (Collaboration et al., 25 Aug 2025). The radiated energy is approximately

$22.4$6

in the source frame, with the credible interval broadened by the same model systematics that affect the component masses and final mass (Collaboration et al., 25 Aug 2025).

GW231028_153006 is described as among the most massive BBHs in O4a, with strongly positive $22.4$7 and significant support for asymmetric masses (Collaboration et al., 25 Aug 2025). The event is also among those whose remnant likely exceeds $22.4$8 with high probability, although the combined-table value near $22.4$9 sits close to that threshold because of model-dependent systematics (Collaboration et al., 25 Aug 2025). This places the event near the intermediate-mass black hole boundary in remnant mass and makes it relevant for discussions of heavy-BBH formation channels (Collaboration et al., 25 Aug 2025).

A plausible implication is that GW231028_153006 is especially informative for testing the interplay between high total mass, large aligned spin, and mass asymmetry in BBH formation scenarios. That implication remains interpretive; the concrete catalog statements are limited to its high mass, positive $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$0, unequal masses, and remnant properties (Collaboration et al., 25 Aug 2025).

5. Ringdown spectroscopy and the overtone claim

A dedicated time-domain ringdown analysis models the post-peak signal as a coherent sum of damped quasinormal modes (QNMs) (Wang, 10 Sep 2025): $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$1 with

$< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$2

For a Kerr black hole, the mode frequencies and damping times are determined by the remnant mass and spin,

$< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$3

The analysis starts the ringdown at $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$4 with $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$5 in steps of $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$6 (Wang, 10 Sep 2025). For GW231028, the network polarization peak is at $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$7 GPS and the Hanford polarization peak is at $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$8 GPS (Wang, 10 Sep 2025). With catalog value $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$9, one has $21.0$0, so $21.0$1 corresponds to about $21.0$2 after the peak (Wang, 10 Sep 2025). The sky location and polarization angle are fixed to the IMR maximum-likelihood values $21.0$3 (Wang, 10 Sep 2025).

Inference is performed with a time-domain $21.0$4-statistic that analytically maximizes over linear amplitude and phase coefficients. Assuming stationary Gaussian noise, the multi-detector likelihood is

$21.0$5

and the maximization over linear parameters yields

$21.0$6

where $21.0$7 is the vector of data–basis inner products and $21.0$8 is the normal matrix (Wang, 10 Sep 2025). Bayesian model comparison then uses

$21.0$9

The principal result is that the two-mode model containing the fundamental $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$0 and first overtone $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$1 is favored over the $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$2-only hypothesis (Wang, 10 Sep 2025). At $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$3,

$< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$4

which is classified as decisive under Kass–Raftery criteria (Wang, 10 Sep 2025). The support peaks earlier, at $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$5, with $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$6, but the study adopts the more conservative $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$7 start to remain safely within the linear perturbative regime (Wang, 10 Sep 2025). The network ringdown SNR is approximately $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$8 at $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$9 and decays to approximately $21.9$0 by $21.9$1 (Wang, 10 Sep 2025).

Alternative two-mode combinations were explored, adding one of $21.9$2 to the $21.9$3 mode (Wang, 10 Sep 2025). Although $21.9$4 sometimes shows slightly higher raw evidence, it fails a physical consistency check because it does not produce stable, meaningful posteriors for $21.9$5 across start times (Wang, 10 Sep 2025). By contrast, $21.9$6 yields mass–spin posteriors consistent with independent full IMR analyses based on NRSur7dq4 and SEOBNRv5PHM and is therefore identified as the physically preferred model (Wang, 10 Sep 2025). The preferred two-mode analysis at $21.9$7 gives

$21.9$8

at $21.9$9 credibility (Wang, 10 Sep 2025).

The same study performs a no-hair test by allowing fractional deviations in the overtone frequency and damping time,

$< 1\,\mathrm{yr}^{-1}$00

for which Kerr general relativity predicts $< 1\,\mathrm{yr}^{-1}$01 and $< 1\,\mathrm{yr}^{-1}$02 (Wang, 10 Sep 2025). At $< 1\,\mathrm{yr}^{-1}$03 the constraints are

$< 1\,\mathrm{yr}^{-1}$04

consistent with general relativity (Wang, 10 Sep 2025). The paper characterizes this as the first decisive overtone detection firmly within the linear regime (Wang, 10 Sep 2025).

6. Rapid-alert classification and follow-up context

GW231028_153006 also appears indirectly in the rapid-alert literature through GWSkyNet-Multi II, a deep-learning model designed to provide real-time classification of candidate events using LVK public-alert localization products (Raza et al., 1 Feb 2025). The model outputs normalized probabilities for the four LVK categories—glitch, BBH, NSBH, and BNS—via a softmax layer,

$< 1\,\mathrm{yr}^{-1}$05

and quantifies uncertainty using an ensemble of $< 1\,\mathrm{yr}^{-1}$06 models trained on randomized train/validation/test splits (Raza et al., 1 Feb 2025). It ingests nine summary features derived from BAYESTAR rapid alerts: sky localization area, $< 1\,\mathrm{yr}^{-1}$07D localization volume, mean distance, distance uncertainty, Log BCI, Log BSN clipped at $< 1\,\mathrm{yr}^{-1}$08, and a three-element detector observing-state vector (Raza et al., 1 Feb 2025).

For significant O4 multi-detector alerts issued through December 2024, GWSkyNet-Multi II agrees with the latest LVK updated classifications for $< 1\,\mathrm{yr}^{-1}$09 events, or $< 1\,\mathrm{yr}^{-1}$10 consistency (Raza et al., 1 Feb 2025). Its intended operational use is triage: high BNS or NSBH probability motivates electromagnetic follow-up, whereas BBH or glitch classifications suggest lower priority for scarce follow-up resources (Raza et al., 1 Feb 2025).

GW231028_153006 is not listed in the paper’s table of “select significant O4 events,” and the event is not otherwise referenced in the main text, figures, or appendices (Raza et al., 1 Feb 2025). The authors explicitly omit events for which both LVK and GWSkyNet-Multi II were confidently BBH with $< 1\,\mathrm{yr}^{-1}$11 probability (Raza et al., 1 Feb 2025). This strongly suggests that GW231028_153006 fell into that category, but the paper does not provide per-event model probabilities, uncertainties, FAR, sky area, distance, network SNR, or LVK metadata for this timestamp (Raza et al., 1 Feb 2025). A plausible implication is that, at the rapid-alert stage, the event was probably a straightforward low-priority target for electromagnetic follow-up because of its likely high-confidence BBH classification; this remains an inference rather than an explicit per-event result (Raza et al., 1 Feb 2025).

Taken together, the catalog, ringdown, and rapid-classification studies present GW231028_153006 as a loud O4a BBH merger with high total mass, positive aligned spin, substantial waveform-model systematics in full-IMR inference, and unusually strong post-merger content that enables decisive two-mode ringdown spectroscopy (Collaboration et al., 25 Aug 2025, Wang, 10 Sep 2025, Raza et al., 1 Feb 2025).