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

TL;DR

This paper presents a comprehensive analysis of the GW200105 neutron star-black hole merger, employing advanced waveform models that include eccentricity and spin precession, confirming the presence of eccentricity and exploring its effects on merger dynamics.

Contribution

It introduces the first physically complete waveform model for this event that incorporates eccentricity, spin precession, and higher-order modes across all stages of the merger.

Findings

Eccentricity is supported in the signal with zero eccentricity excluded at 99% credible interval.

The mass ratio estimate aligns more closely with initial LIGO-Virgo-KAGRA analysis.

Multimodal eccentricity posterior distribution is observed and investigated.

Abstract

GW200105_162426 is the first neutron star-black hole merger to be confidently confirmed through either gravitational-wave or electromagnetic observations. Although initially analyzed after detection, the event has recently gained renewed attention following a study [Morras et al. arXiv:2503.15393] that employed a post-Newtonian inspiral-only waveform model and reported strong evidence for orbital eccentricity. In this work, we perform a detailed analysis of GW200105 using state-of-the-art effective-one-body waveform models. Importantly, we present the first study of this event utilizing a physically complete model that incorporates both orbital eccentricity and spin precession across the full inspiral, merger, and ringdown stages, along with higher-order gravitational wave modes. Our results support the presence of eccentricity in the signal, with zero eccentricity excluded from the 99% credible interval, but yielding a mass ratio closer to the original LIGO-Virgo-KAGRA analysis, differing from the findings of [Morras et al. arXiv:2503.15393]. Additionally, similar to a previous eccentric-only analysis [de Lluc Planas et al. Astrophys. J. 995, 47 (2025).], we observe a multimodal structure in the eccentricity posterior distribution. We conduct targeted investigations to understand the origin of this multimodality and complement our analysis with numerical relativity simulations to examine how the inclusion of eccentricity impacts the merger dynamics.