GW230814: investigation of a loud gravitational-wave signal observed with a single detector (2509.07348v1)

Abstract: GW230814 was detected by the LIGO Livingston observatory with a signal-to-noise ratio of 42.4 making it the loudest gravitational-wave signal in the GWTC-4.0 catalog. The source is consistent with a binary black hole coalescence similar to those in the previously observed population, with component masses $m_1 = 33.7^{+2.9}_{-2.2}$ $\mathrm{M}\odot$, $m_2 = 28.2^{+2.2}{-3.1} M_\odot$, and small effective inspiral spin $\chi_{\mathrm{eff}}= -0.01^{+0.06}_{-0.07}$. The high signal-to-noise ratio enabled us to confidently detect an $\ell = |m| = 4$ mode in the inspiral signal for the first time, and enables a range of tests of consistency between theoretical predictions and the observed waveform. Most of these tests show good agreement with expectations from general relativity. However, a few indicate deviations, particularly in the ringdown part of the signal. We find that deviations comparable to those observed can be obtained from similar simulated signals based on general relativity and detector noise effects. Therefore, the apparent deviations do not provide evidence for a violation of general relativity. The observation of GW230814 demonstrates that while the unprecedented sensitivity of the detectors enable highly significant detections with a single observatory, drawing robust inferences about fundamental physics remains limited without data from a multiple observatory network.

• The paper presents a detailed analysis of GW230814, the highest-SNR BBH merger observed with a single detector, confirming its signal significance with rigorous statistical validation.
• It employs advanced parameter estimation using multiple waveform models via Bilby, constraining masses, spins, and viewing angles, and marking the first confident detection of the l=|m|=4 mode.
• Tests of general relativity, including FTI, TIGER, and ringdown analyses, highlight the impact of waveform systematics and noise fluctuations, emphasizing the need for multi-detector observations.

GW230814: Analysis of a High-SNR Gravitational-Wave Event Observed with a Single Detector

Overview and Event Detection

The paper presents a comprehensive investigation of GW230814, a binary black hole (BBH) coalescence detected exclusively by the LIGO Livingston Observatory (LLO) with a matched-filter signal-to-noise ratio (SNR) of 42.4, the highest among compact binary coalescences (CBCs) in the GWTC-4.0 catalog. The event was identified in low latency by the GstLAL pipeline with an inverse false alarm rate (IFAR) exceeding $10^5$ years, and subsequently confirmed by offline analyses. The absence of corroborating data from other detectors (Hanford, Virgo, KAGRA) necessitated rigorous vetting of data quality and statistical significance, which the authors demonstrate is robust for this event.

Figure 1: Time-frequency representation and whitened time-domain strain with Bayesian reconstructions from LIGO Livingston observations of GW230814.

Source Characterization and Parameter Estimation

Parameter estimation (PE) was performed using Bilby with four waveform models: SEOBNRv5PHM, IMRPhenomXPHM, IMRPhenomXO4a, and NRSur7dq4. The inferred source properties are consistent across models, with component masses $m_1 = 33.7^{+2.9}_{-2.7}\,M_\odot$, $m_2 = 28.2^{+2.2}_{-2.1}\,M_\odot$, mass ratio $q = 0.84^{+0.12}_{-0.13}$, and chirp mass $\mathcal{M} = 26.8^{+0.9}_{-0.9}\,M_\odot$. The effective inspiral spin is tightly constrained to $\chi_{\rm eff} = -0.01^{+0.06}_{-0.06}$, with both component spins found to be small and oppositely aligned with respect to the orbital angular momentum.

Figure 2: Joint posterior distributions for the two component masses, showing high measurement precision due to the event's SNR.

Figure 3: Posterior probability density estimates for spin magnitudes and tilt angles, indicating small, oppositely aligned spins for both components.

The luminosity distance is estimated as $D_L = 289^{+183}_{-120}$ Mpc, corresponding to a redshift $z = 0.06^{+0.04}_{-0.03}$. The final remnant mass and spin are $M_f = 62.7^{+1.5}_{-1.5}\,M_\odot$ and $\chi_f = 0.68^{+0.02}_{-0.02}$, respectively.

Higher-Order Mode Detection and Viewing Geometry

The high SNR enabled the first confident detection of the $\ell=|m|=4$ mode in the inspiral phase of a CBC, with subdominant angular modes ($\ell=2,3,4$) contributing significantly to the signal. This allowed for constraints on the inclination angle $\theta_{JN}$, favoring edge-on viewing geometries, despite the single-detector configuration.

Tests of General Relativity

A suite of tests was performed to probe the consistency of the signal with general relativity (GR), including:

• Flexible Theory-Independent (FTI) and TIGER tests: These constrain deviations in post-Newtonian (PN) coefficients during the inspiral. The bounds on higher-order PN terms surpass those from previous events such as GW170817, with all deviation parameters consistent with zero.

  Figure 4: Bounds from FTI and TIGER tests on PN coefficients, showing improved constraints over previous events.

• TIGER merger-ringdown analysis: Posterior distributions for merger-ringdown calibration coefficients ($\delta \hat{c}_i$) are shifted away from the GR prediction, with GR quantiles as low as $0.2\%$ for some coefficients. However, Bayes factors do not provide statistically significant evidence for GR violation.

  Figure 5: TIGER test results for intermediate and merger-ringdown coefficients, with GR prediction outside the 90% credible intervals for some parameters.

• Inspiral-Merger-Ringdown Consistency Test (IMRCT): The final mass and spin inferred from the inspiral and post-inspiral phases are consistent within the 47% credible region, indicating no significant inconsistency.

  Figure 6: IMRCT results showing consistency between inspiral and post-inspiral estimates of remnant properties.

• Ringdown Analyses (pSEOBNR, pyRing, QNMRF): The pSEOBNR analysis finds a fractional deviation in the damping time of the $(2,2,0)$ quasi-normal mode (QNM), $\delta \hat{\tau}_{220} = -0.3^{+0.15}_{-0.12}$, with the GR value excluded at the $99\%$ credible level. The pyRing and QNMRF analyses corroborate a reduced damping time, but Bayesian model selection does not favor non-GR models.

  Figure 7: 90% credible regions for fractional deviations in frequency and damping time of the $(2,2,0)$ QNM.

  

  Figure 8: Inferred frequencies and damping times from various ringdown analyses, showing tension with GR predictions for the $(2,2,0)$ mode.

Investigation of Apparent Deviations

The authors systematically investigate the origin of the observed post-merger deviations:

• Waveform Systematics: Faithfulness tests against numerical relativity (NR) simulations indicate that the NRSur model achieves unfaithfulness below the threshold required by the event's SNR for $>99\%$ of relevant simulations, but other models (SEOBNR, XPHM, XO4a) approach or exceed the threshold, suggesting waveform systematics could contribute to the observed deviations.

  Figure 9: SNR-weighted unfaithfulness of waveform models with respect to NR simulations, showing NRSur's superior accuracy.

Zero-noise injections of NRSur and NR waveforms analyzed with pSEOBNR reproduce similar deviations in the damping time, supporting the hypothesis that waveform systematics can mimic GR violations.

• Detector Noise Fluctuations: Injections of GR waveforms into real LLO noise yield deviations comparable to those observed in GW230814 in up to 50% of cases, indicating that statistical noise fluctuations are a plausible explanation.

  Figure 10: Comparison of whitened data and reconstructed waveforms, highlighting a segment with faster amplitude decay than predicted by GR models.

  

  Figure 11: Fisher information accumulation for the damping time parameter, showing sensitivity to the time segment with anomalous decay.

  

  Figure 12: Distribution of Bayes factors and credible intervals for pSEOBNR analyses on simulated signals, demonstrating that real noise can reproduce the observed deviation.

• Subdominant Modes and Lensing: No evidence is found for additional ringdown modes or gravitational lensing effects as the source of the deviation.

Implications and Limitations

The analysis demonstrates that single-detector events with high SNR can yield precise measurements of source parameters and enable detailed tests of GR, including constraints on higher-order modes and inclination angles. However, the lack of multi-detector data severely limits the ability to distinguish between genuine physical deviations and artifacts arising from waveform systematics or detector noise. The authors emphasize that robust tests of fundamental physics require a global network of gravitational-wave observatories.

Conclusion

GW230814 represents the loudest BBH coalescence in GWTC-4.0, enabling the first confident detection of an $\ell=4$ mode and stringent tests of GR. While the inspiral phase is consistent with GR, mild inconsistencies in the post-merger damping time are observed. These are not statistically significant and can be reproduced by waveform systematics and detector noise fluctuations. The paper highlights the necessity of multi-detector observations for unambiguous tests of fundamental physics and sets a benchmark for future single-detector analyses.