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

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.