GW200105: A detailed study of eccentricity in the neutron star–black hole binary

Title: GW200105: A detailed study of eccentricity in the neutron star–black hole binary

Bibcode: 2026PhRvD.113b4018J

Highlight: GW200105_162426 is the first neutron star–black hole (NSBH) merger to be confidently confirmed through either gravitational-wave or electromagnetic observations. A prior study by Morras et al. reported strong evidence for orbital eccentricity using an inspiral-only, post-Newtonian waveform model. In this work, the authors perform a more rigorous analysis using state-of-the-art effective-one-body (EOB) waveform models — the first study of this event to include both orbital eccentricity and spin precession across the full inspiral, merger, and ringdown, alongside higher-order gravitational-wave modes.

The results confirm that eccentricity is present in the signal — zero eccentricity is excluded at the 99% credible interval. However, the inferred mass ratio aligns more closely with the original LIGO-Virgo-KAGRA analysis, differing from the Morras et al. findings. The authors also identify a multimodal structure in the eccentricity posterior, consistent with an earlier eccentric-only analysis, and investigate its origin using targeted analyses and numerical relativity simulations.

For experts: The key advance here is the use of a physically complete EOB model that self-consistently couples orbital eccentricity and spin precession across the full signal, including higher-order modes. This goes beyond the inspiral-only post-Newtonian treatment of Morras et al. and the eccentric-only analysis of de Lluc Planas et al. (ApJ 995, 47, 2025). The authors find that the eccentricity posterior exhibits multimodal structure and conduct numerical relativity simulations to examine how eccentricity affects the merger dynamics — an important probe of formation channel.