---
title: 'GW231028: Massive BBH Merger & Ringdown Overtone'
url: https://www.emergentmind.com/topics/gw231028_153006
type: topic
---

# GW231028: Massive BBH Merger & Ringdown Overtone

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 $< 1\,\mathrm{yr}^{-1}$ [2508.18082]. 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 [2508.18082]. 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 [2509.08657].

## 1. Detection and catalog placement

GW231028_153006 has UTC coalescence time 2023-10-28 15:30:06, as encoded in the event name [2508.18082]. 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 [2508.18082]. It passed validation, is included in the parameter-estimation set, and is classified as a BBH [2508.18082].

| 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 | $< 1.0\times10^{-5}\,\mathrm{yr}^{-1}$ | $21.9$ |

All four pipelines report $p_{\rm astro} > 0.99$ [2508.18082]. Representative per-detector signal-to-noise ratios vary slightly across pipelines: PyCBC gives $(\rho_H,\rho_L)=(11.8,18.4)$, GstLAL gives $(12.0,17.2)$, MBTA gives $(12.5,18.0)$, and CWB-BBH gives $(14.1,17.4)$, with the network value computed for two detectors as $\rho_{\rm net}=\sqrt{\rho_H^2+\rho_L^2}$ [2508.18082].

The catalog places GW231028_153006 among the O4a compact binary coalescence candidates identified by at least one search algorithm with $p_{\rm astro}\geq 0.5$ and not vetoed during event validation [2508.18082]. 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 [2508.18082].

## 2. Source properties

Under the default LVK “Planck 2015” cosmology, the source-frame total mass is reported as $M=152^{+29}_{-14}\,M_\odot$, with mass ratio
$q=m_2/m_1=0.63^{+0.33}_{-0.35}$ and chirp mass
$\mathcal{M}=63^{+13}_{-10}\,M_\odot$ [2508.18082]. The catalog uses the standard relations
\[
\mathcal{M}=\frac{(m_1m_2)^{3/5}}{(m_1+m_2)^{1/5}},\qquad
\eta=\frac{m_1m_2}{(m_1+m_2)^2}=\frac{q}{(1+q)^2},
\]
and notes that $\eta \approx 0.24$ with an approximate $90\%$ range $\approx 0.17$–$0.25$, derived from the quoted $q$ interval [2508.18082].

Spin inference indicates a distinctly positive aligned component. The effective aligned spin is
\[
\chi_{\rm eff}=\frac{m_1\chi_1\cos\theta_1+m_2\chi_2\cos\theta_2}{m_1+m_2}
=0.4^{+0.2}_{-0.2},
\]
while the combined “MIXED” posterior gives $\chi_p \approx 0.44^{+0.34}_{-0.23}$ for the effective precession parameter [2508.18082]. The primary spin is likely large, with $X_1>0.8$ with $\approx 47\%$ probability, although tilt angles are not tightly constrained and the posterior remains broadly consistent with a net aligned-spin configuration [2508.18082].

The luminosity distance is approximately $D_L \approx 2.2^{+0.9}_{-0.9}\,\mathrm{Gpc}$ and the redshift is approximately $z \approx 0.53^{+0.32}_{-0.26}$ [2508.18082]. The catalog emphasizes the standard two-detector degeneracy between $D_L$ and inclination, so inclination is not tightly constrained [2508.18082]. Likewise, no per-event sky area is quoted in the catalog summary; with only H and L observing in O4a, $90\%$ localizations are generally $\mathcal{O}(10^2$–$10^3)\,\mathrm{deg}^2$ [2508.18082].

The event exhibits broad and model-dependent component-mass posteriors. The combined posterior implies medians near $m_1 \approx 93\,M_\odot$ and $m_2 \approx 59\,M_\odot$, but the credible ranges are broad and model-dependent, and the catalog does not highlight single-model medians for $m_1$ and $m_2$ because of systematic variations and multimodality [2508.18082].

## 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 [2508.18082]. Detector-frame and source-frame masses are related by
\[
m^{\rm det}=(1+z)m^{\rm src},
\]
with redshift conversion determined by the adopted Planck 2015 cosmology [2508.18082].

For BBHs, including GW231028_153006, the catalog combines equal numbers of posterior samples from four waveform families: IMRPhenomXPHM_SpinTaylor, SEOBNRv5PHM, NRSUR7DQ4, and IMRPhenomXO4a [2508.18082]. The reported “MIXED” results are intended to mitigate model systematics, which are particularly relevant for this event [2508.18082]. Both higher-order modes and precession are treated by the default BBH models, and $\chi_p$ is explicitly inferred [2508.18082].

A salient feature of GW231028_153006 is the presence of significant systematic variations across waveform models for masses and spins [2508.18082]. IMRPhenomXO4a shows a secondary mode at high $\chi_p$, and none of the four models closely agree on the exact component masses [2508.18082]. 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 [2508.18082]. Cross-checks with minimally modeled reconstructions show no evidence for missing waveform content beyond standard compact-binary-coalescence physics: BAYESWAVE gives on-source overlap $0.99$, off-source $0.97\pm0.02$, $p=0.99$; CWB-2G gives on-source $0.97$, off-source $0.96\pm0.01$, $p=0.74$; and CWB-BBH gives on-source $0.97$, off-source $0.97\pm0.01$, $p=0.42$ [2508.18082].

## 4. Remnant properties and astrophysical significance

The catalog reports a remnant black hole mass of approximately $M_f \approx 98\,M_\odot$ and final dimensionless spin $a_f \approx 0.70^{+0.09}_{-0.05}$ [2508.18082]. The radiated energy is approximately
\[
E_{\rm rad}\approx (m_1+m_2)-M_f \approx 54\,M_\odot c^2
\]
in the source frame, with the credible interval broadened by the same model systematics that affect the component masses and final mass [2508.18082].

GW231028_153006 is described as among the most massive BBHs in O4a, with strongly positive $\chi_{\rm eff}\approx 0.4$ and significant support for asymmetric masses [2508.18082]. The event is also among those whose remnant likely exceeds $100\,M_\odot$ with high probability, although the combined-table value near $98\,M_\odot$ sits close to that threshold because of model-dependent systematics [2508.18082]. 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 [2508.18082].

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 $\chi_{\rm eff}$, unequal masses, and remnant properties [2508.18082].

## 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) [2509.08657]:
\[
h_+(t)+i\,h_\times(t)
=
\sum_{(\ell,m,n)}
{}_{-2}Y_{\ell m}(\iota,\delta)\,
A_{\ell mn}\,
\exp\!\big[i\big(\Omega_{\ell mn}(t-t_0)+\phi_{\ell mn}\big)\big],
\]
with
\[
\Omega_{\ell mn}=2\pi f_{\ell mn}+i/\tau_{\ell mn}.
\]
For a Kerr black hole, the mode frequencies and damping times are determined by the remnant mass and spin,
\[
f_{\ell mn}=f_{\ell mn}(M_f,\chi_f),\qquad
\tau_{\ell mn}=\tau_{\ell mn}(M_f,\chi_f).
\]

The analysis starts the ringdown at $t_0=t_c^{\rm pol}+\Delta t$ with $\Delta t\in[8,20]\,M$ in steps of $2\,M$ [2509.08657]. For GW231028, the network polarization peak is at $t_c^{\rm pol}=1382542224.18917$ GPS and the Hanford polarization peak is at $t_c^{\rm pol}(\mathrm{H1})=1382542224.20303$ GPS [2509.08657]. With catalog value $M_f^z\approx246.4\,M_\odot$, one has $1\,M\approx1.2\,\mathrm{ms}$, so $\Delta t=10\,M$ corresponds to about $12\,\mathrm{ms}$ after the peak [2509.08657]. The sky location and polarization angle are fixed to the IMR maximum-likelihood values $(\mathrm{RA},\mathrm{DEC},\psi)=(0.04,-0.10,1.26)$ [2509.08657].

Inference is performed with a time-domain $F$-statistic that analytically maximizes over linear amplitude and phase coefficients. Assuming stationary Gaussian noise, the multi-detector likelihood is
\[
\ln \mathcal{L}(d\mid\theta)
=
-\frac{1}{2}\sum_k \big(d_k-h_k(\theta)\,\big|\,d_k-h_k(\theta)\big)_k,
\]
and the maximization over linear parameters yields
\[
\hat a=M^{-1}x,\qquad 2\mathcal{F}=x^\top M^{-1}x,
\]
where $x$ is the vector of data–basis inner products and $M$ is the normal matrix [2509.08657]. Bayesian model comparison then uses
\[
\mathcal{B}^{\mathbb{M}_1}_{\mathbb{M}_0}
=
\frac{\mathcal{Z}_{\mathbb{M}_1}}{\mathcal{Z}_{\mathbb{M}_0}}.
\]

The principal result is that the two-mode model containing the fundamental $220$ and first overtone $221$ is favored over the $220$-only hypothesis [2509.08657]. At $\Delta t=10\,M$,
\[
\mathcal{B}^{220+221}_{220}\approx193,
\]
which is classified as decisive under Kass–Raftery criteria [2509.08657]. The support peaks earlier, at $\Delta t=8\,M$, with $\log_{10}\mathcal{B}^{220+221}_{220}\approx5.2$, but the study adopts the more conservative $10\,M$ start to remain safely within the linear perturbative regime [2509.08657]. The network ringdown SNR is approximately $10.5$ at $\Delta t=10\,M$ and decays to approximately $5.9$ by $\Delta t=20\,M$ [2509.08657].

Alternative two-mode combinations were explored, adding one of $\{221,200,210,320,330,430,440\}$ to the $220$ mode [2509.08657]. Although $220+210$ sometimes shows slightly higher raw evidence, it fails a physical consistency check because it does not produce stable, meaningful posteriors for $(M_f,\chi_f)$ across start times [2509.08657]. By contrast, $220+221$ yields mass–spin posteriors consistent with independent full IMR analyses based on NRSur7dq4 and SEOBNRv5PHM and is therefore identified as the physically preferred model [2509.08657]. The preferred two-mode analysis at $\Delta t=10\,M$ gives
\[
M_f^z=243.0^{+22.7}_{-22.7}\,M_\odot,\qquad
\chi_f=0.80^{+0.07}_{-0.11}
\]
at $90\%$ credibility [2509.08657].

The same study performs a no-hair test by allowing fractional deviations in the overtone frequency and damping time,
\[
f_{221}\to f_{221}(1+\delta f_{221}),\qquad
\tau_{221}\to \tau_{221}(1+\delta\tau_{221}),
\]
for which Kerr general relativity predicts $\delta f_{221}=0$ and $\delta\tau_{221}=0$ [2509.08657]. At $\Delta t=10\,M$ the constraints are
\[
\delta f_{221}=0.06^{+0.27}_{-0.25},\qquad
\delta\tau_{221}=0.47^{+0.48}_{-0.85},
\]
consistent with general relativity [2509.08657]. The paper characterizes this as the first decisive overtone detection firmly within the linear regime [2509.08657].

## 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 [2502.00297]. The model outputs normalized probabilities for the four LVK categories—glitch, BBH, NSBH, and BNS—via a softmax layer,
\[
p_k=\frac{\exp(z_k)}{\sum_j \exp(z_j)},
\]
and quantifies uncertainty using an ensemble of $20$ models trained on randomized train/validation/test splits [2502.00297]. It ingests nine summary features derived from BAYESTAR rapid alerts: sky localization area, $3$D localization volume, mean distance, distance uncertainty, Log BCI, Log BSN clipped at $100$, and a three-element detector observing-state vector [2502.00297].

For significant O4 multi-detector alerts issued through December 2024, GWSkyNet-Multi II agrees with the latest LVK updated classifications for $168/180$ events, or $93\%$ consistency [2502.00297]. 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 [2502.00297].

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 [2502.00297]. The authors explicitly omit events for which both LVK and GWSkyNet-Multi II were confidently BBH with $>90\%$ probability [2502.00297]. 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 [2502.00297]. 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 [2502.00297].

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 [2508.18082] [2509.08657] [2502.00297].

Source: https://www.emergentmind.com/topics/gw231028_153006