Analyzing GW231109 235456 and understanding its potential implications for population studies, nuclear physics, and multi-messenger astronomy

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. Analyzing the component masses, we find that GW231109 235456, if astrophysical, supports the proposed double Gaussian mass distribution of neutron stars. 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 but within reach of observatories such as LSST and Roman. In addition, we provide constraints on the equation of state from the candidate GW231109 235456 alone, as well as combined with GW170817 and GW190425. In our projections for the future, we simulate a similar event using the planned next generation of ground-based gravitational-wave detectors. Our findings indicate that we can constrain the tidal deformability of a 1.4 M neutron star to about 10%, depending on the specific designs and network considered.