Analyzing GW231109_235456 and understanding its potential implications for population studies, nuclear physics, and multi-messenger astronomy

TL;DR

This paper analyzes the sub-threshold binary neutron star candidate GW231109_235456 from LVK O4a using state-of-the-art BNS waveform models and varied priors to probe how population assumptions and tidal physics shape inferred source properties. It finds that GW231109 is most consistent with a double-Gaussian neutron star mass distribution, and that the merger likely produced a prompt black-hole remnant with a potentially very dim kilonova; EOS constraints from GW231109 alone are weak but become informative when combined with GW170817 and GW190425, yielding $R_{1.4}=12.1^{+1.1}_{-1.2}$ km at 90% credibility. The study also projects how third-generation detectors (ET and CE) would sharpen neutron-star radius constraints to about 300–400 m, and demonstrates how future observations could dramatically improve multi-messenger constraints on dense matter. Overall, the work clarifies the role of sub-threshold events in population studies, EOS inference, and multi-messenger astronomy, while outlining caveats related to SNR and waveform systematics.

Abstract

We study the gravitational-wave trigger GW231109_235456, a sub-threshold binary neutron star merger candidate observed in the first part of the fourth observing run of the LIGO-Virgo-KAGRA collaboration. Assuming the trigger is of astrophysical origin, we analyze it using state-of-the-art waveform models and investigate the robustness of the inferred source parameters under different prior choices in Bayesian inference. We assess the implications for population studies, nuclear physics, and multi-messenger astronomy. Analysing the component masses, we find that GW231109_235456 supports the proposed double Gaussian mass distribution of neutron star masses. Moreover, we find that the remnant most likely collapsed promptly to a black hole and that, because of the large distance, a possible kilonova connected to the merger was noticeably dimmer than AT2017gfo. In addition, we provide constraints on the equation of state from GW231109_235456 alone, as well as combined with GW170817 and GW190425. In our projections for the future, we simulate a similar event using the upcoming generation of gravitational-wave detectors. Our findings indicate that we can constrain the neutron star radius with an accuracy of 400 meters using the Einstein Telescope alone, or 300 meters when combined with the Cosmic Explorer, both at 90% credibility.

Figures (7)

• Figure 1: Source-frame component masses of low-mass GW events that most likely contain at least one NS, namely, GW170817 LIGOScientific:2017vwq, GW190425 LIGOScientific:2020aai, GW230529 LIGOScientific:2024elc, and GW231109_235456 (GW231109). GW231109's individual masses lie between those of GW170817 and GW190425. The gray shade in the 1D panels shows the transition from NS to BH masses: the color opacity corresponds to the cumulative density function of the TOV mass posterior, based on the uncertainty in the TOV mass inferred later in this work from measurements of heavy pulsars (see Sec. \ref{['sec:eos']} for details).
• Figure 2: Posterior on chirp mass $\mathcal{M}_c$, mass ratio $q$, effective spin $\chi_{\rm eff}$, and mass-weighted tidal deformability $\tilde{\Lambda}$ of the GW inference using the default mass priors, i.e., uniform in detector-frame component masses. Left panel: $\Lambda_i$ are sampled uniformly in the range $[0, 5000]$. Right panel: $\Lambda_i$ are determined by the EOS sampled on-the-fly, with samples where the primary mass exceeds the TOV mass being discarded. The light (dark) shading indicates the $68\%$ ($95\%$) credible area. The median values of the recovered parameters, together with the $90\%$ credible intervals, are reported above the marginalized posterior distributions.
• Figure 3: Comparison between priors (dashed lines) from populations and corresponding posteriors (solid lines) of the source-frame component masses, for the four mass priors considered.
• Figure 4: The lightcurves presented with the median values, along with the 90% credible interval, measured in AB magnitudes across various photometric bandpasses. These observations are compared with data from AT2017gfo LIGOScientific:2017pwlAndreoni:2017ppdCoulter:2017wyaLipunov:2017dwdShappee:2017zlyTanvir:2017pwsJ-GEM:2017tyx.
• Figure 5: Posterior on parameters from a simulated source similar to GW231109 observed with ET and a network of ET with CE. The black lines indicate the injected values. The median values of the recovered parameters, together with the $90\%$ credible intervals, are reported above the marginalized posterior distributions.