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GW231109_235456: A Sub-threshold Binary Neutron Star Merger in the LIGO-Virgo-KAGRA O4a Observing Run?

Wanting Niu, Carl-Johan Haster, Jolien D. E. Creighton, Chad Hanna, Shomik Adhicary, Pratyusava Baral, Amanda Baylor, Bryce Cousins, Heather Fong, Yun-Jing Huang, Rachael Huxford, Prathamesh Joshi, James Kennington, Alvin K. Y. Li, Ryan Magee, Duncan Meacher, Cody Messick, Soichiro Morisaki, Cort Posnansky, Surabhi Sachdev, Shio Sakon, Urja Shah, Divya Singh, Ron Tapia, Leo Tsukada, Aaron Viets, Zach Yarbrough, Noah Zhang

TL;DR

This study develops a binary neutron star (BNS) targeted search in LVK O4a data by incorporating a redshift-corrected Galactic DNS population model to boost sensitivity to DNS mergers. It builds a DNS-specific template bank and ranks candidates with a population-informed prior, reporting a subthreshold trigger GW231109_235456 with a false-alarm rate reduced to about $0.1$ yr$^{-1}$ after a fivefold trials correction, and performs Bayesian parameter estimation to infer component masses and tidal properties. The results indicate masses consistent with known BNS systems, provide constraints on the tidal deformability, and yield a BNS merger-rate estimate that agrees with prior LVK population analyses. The work demonstrates that population-tailored priors can significantly enhance subthreshold searches and motivates integrating DNS priors into standard pipelines for future observations and multi-messenger follow-up.

Abstract

We present a subthreshold search for gravitational-wave inspirals from binary neutron stars using data from the first part of the fourth observing run of the LIGO-Virgo-KAGRA Collaboration. To enhance sensitivity to this targeted population, we incorporate a redshift-corrected population model informed by radio observations of Galactic double neutron star systems. The search identifies a significant trigger with a false-alarm rate of one per fifty years and a network signal-to-noise ratio of 9.7. This trigger was first reported in low-latency processing as S231109ci and subsequently listed in the GWTC-4.0 catalog as GW231109_235456, a subthreshold candidate. Accounting for a trials factor of five arising from four previous searches in GWTC-4.0 and this new search, the false-alarm rate of the candidate is approximately one per ten years. If the event is of astrophysical origin, the inferred source properties indicate component masses of 1.40 to 2.24 solar masses for the primary and 0.97 to 1.49 solar masses for the secondary, yielding a total mass of 2.95 solar masses with an uncertainty of plus 0.38 and minus 0.07 solar masses. The event is localized to a region of 450 square degrees enclosing ninety percent probability at a luminosity distance of 165 megaparsecs with an uncertainty of plus 70 and minus 69 megaparsecs. Assuming the signal arises from a binary neutron star merger, we estimate the local merger rate to lie between 53 and 342 per cubic gigaparsec per year.

GW231109_235456: A Sub-threshold Binary Neutron Star Merger in the LIGO-Virgo-KAGRA O4a Observing Run?

TL;DR

This study develops a binary neutron star (BNS) targeted search in LVK O4a data by incorporating a redshift-corrected Galactic DNS population model to boost sensitivity to DNS mergers. It builds a DNS-specific template bank and ranks candidates with a population-informed prior, reporting a subthreshold trigger GW231109_235456 with a false-alarm rate reduced to about yr after a fivefold trials correction, and performs Bayesian parameter estimation to infer component masses and tidal properties. The results indicate masses consistent with known BNS systems, provide constraints on the tidal deformability, and yield a BNS merger-rate estimate that agrees with prior LVK population analyses. The work demonstrates that population-tailored priors can significantly enhance subthreshold searches and motivates integrating DNS priors into standard pipelines for future observations and multi-messenger follow-up.

Abstract

We present a subthreshold search for gravitational-wave inspirals from binary neutron stars using data from the first part of the fourth observing run of the LIGO-Virgo-KAGRA Collaboration. To enhance sensitivity to this targeted population, we incorporate a redshift-corrected population model informed by radio observations of Galactic double neutron star systems. The search identifies a significant trigger with a false-alarm rate of one per fifty years and a network signal-to-noise ratio of 9.7. This trigger was first reported in low-latency processing as S231109ci and subsequently listed in the GWTC-4.0 catalog as GW231109_235456, a subthreshold candidate. Accounting for a trials factor of five arising from four previous searches in GWTC-4.0 and this new search, the false-alarm rate of the candidate is approximately one per ten years. If the event is of astrophysical origin, the inferred source properties indicate component masses of 1.40 to 2.24 solar masses for the primary and 0.97 to 1.49 solar masses for the secondary, yielding a total mass of 2.95 solar masses with an uncertainty of plus 0.38 and minus 0.07 solar masses. The event is localized to a region of 450 square degrees enclosing ninety percent probability at a luminosity distance of 165 megaparsecs with an uncertainty of plus 70 and minus 69 megaparsecs. Assuming the signal arises from a binary neutron star merger, we estimate the local merger rate to lie between 53 and 342 per cubic gigaparsec per year.

Paper Structure

This paper contains 13 sections, 10 equations, 5 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Search Design
  3. DNS Population Model
  4. Search Parameter Space
  5. Search Sensitivity
  6. Results
  7. Source Properties
  8. BNS Merger Rate
  9. Discussion
  10. DNS Catalog
  11. FAR Discrepancy Between Targeted and GWTC-4 Searches
  12. DNS Bank Performance
  13. Sky Map of GW231109_235456

Figures (5)

  • Figure 1: Mass distribution of the DNS population model adopted in this study. The green histogram shows the mass distribution inferred from direct measurements of 13 Galactic DNS systems. The orange and purple histograms represent the simulated primary and secondary masses, respectively, sampled from the source-frame distribution and mapped into the comoving volume.
  • Figure 2: Template bank designed for the DNS-targeted search in the detector-frame $(m_1, m_2)$ parameter space. The color bar indicates the mass-model likelihood Fong2018 derived from the population model described in Sec. \ref{['sec:population']}.
  • Figure 3: Posterior distributions of the binary component masses, $m_1$ and $m_2$, for the high-spin ($|\chi| \leq 0.40$; blue) and low-spin ($|\chi| \leq 0.05$; red) source-inference analyses. The dashed line in the two-dimensional panel denotes the equal-mass limit, $q = 1$. The vertical lines in the one-dimensional panels enclose $90\%$ of the posterior probability and correspond to the ranges reported in Table \ref{['table:detection_candidate']}. The one-dimensional distributions are normalized to have equal maxima.
  • Figure 4: Cumulative distribution of template mismatches. For the DNS-targeted bank, the 90th percentile mismatch between templates and simulated signals is below 0.004, compared with 0.017 for the stellar-mass bank. This corresponds empirically to an expected 2% improvement in sensitivity volume.
  • Figure 5: Sky localization of GW231109_235456 assuming a high-spin prior and a common neutron star equation of state in the source inference. Shaded contours represent credible regions derived from Bayesian parameter estimation using bilby gwtc4-methodsbilby_paperbilby_pipe_paper.