Orbital Eccentricity in a Neutron Star–Black Hole Merger

The observation of gravitational waves (GWs) from merging black holes and neutron stars provides a unique opportunity to discern information about their astrophysical environment. Two signatures that are considered powerful tracers to distinguish between different binary formation channels are general relativistic spin-induced orbital precession and orbital eccentricity. Both effects leave characteristic imprints in the GW signal that can be extracted from observations. To date, neither precession nor eccentricity has been confidently discerned in merging neutron star–black hole binaries. Here we report the measurement of orbital eccentricity in a neutron star–black hole merger. Using, for the first time, a waveform model that incorporates precession and eccentricity, we perform Bayesian inference on the GW event GW200105 and infer a median orbital eccentricity of e₂₀ ∼ 0.145 at an orbital period of 0.1 s, ruling out eccentricities smaller than 0.028 with 99.5% confidence. We find inconclusive evidence for the presence of precession, consistent with previous, noneccentric results, but a more unequal mass ratio. Our result implies a fraction of these binaries will exhibit orbital eccentricity even at small separations, suggesting formation through mechanisms involving dynamical interactions beyond isolated binary evolution. Future observations will reveal the contribution of eccentric neutron star–black hole binaries to the total merger rate across cosmic time.