Impact of eccentricity and higher-modes on neutron star-black hole parameter estimation

Abstract

Detections of gravitational waves from neutron star-black hole systems provide avenues for studying extreme matter, constraining binary formation channels, and testing the nature of compact objects in strong gravity. Eccentric signatures in the signal further enhance this potential by improving parameter estimation and offering clues about binary formation. Because eccentricity is primarily imprinted during the inspiral phase, it is often weakly constrained or missed entirely in binary black hole observations; in contrast, neutron star-black hole systems produce longer in-band signals, enabling more precise measurements of eccentricity and leaving a distinct imprint on parameter inference. In this work, we present a systematic parameter-estimation study exploring the impact of eccentricity on inference using injections simulated with the state-of-the-art eccentric waveform model SEOBNRv5EHM. We find that for systems like GW200105_162426, the measurement precision of eccentricity and correlated parameters improves as eccentricity increases, yielding tighter constraints at larger eccentricities. For the highest eccentricity considered in this study, $e=0.25$, we recover eccentricity with $1\sigma$ uncertainty as low as $4\times10^{-4}$. In addition, the constraints on effective spin $\chi_\mathrm{eff}$ and mass ratio $q$ improve relative to the quasi-circular case by factors of $\sim13$ and $\sim20$, respectively. On the other hand, we find no significant improvement in extrinsic parameters such as luminosity distance and sky localization, suggesting that for systems like GW200105_162426, the additional information provided by eccentricity in this sector is either negligible or degenerate with the information provided by higher-order modes.