Most gravitational wave analyses assume circular orbits. By the time compact binaries enter the LIGO/Virgo frequency band, gravitational radiation has usually circularized the orbit over millions of years. Eccentricity is a fossil — a trace of how the binary formed.
The authors (arXiv:2603.22461) analyze neutron star–black hole mergers from GWTC-4 using waveform models that include eccentricity. The binary neutron star events (GW170817, GW190425) are consistent with zero eccentricity — circular, as expected from isolated stellar evolution. But GW200105, a neutron star–black hole merger, shows significant residual eccentricity at 20 Hz.
Residual eccentricity at 20 Hz means the binary didn't have time to fully circularize. This points away from isolated binary evolution (where the binary forms in situ and circularizes over its long inspiral) and toward dynamical formation — capture in a dense stellar environment, or a hierarchical triple where a third body pumped the eccentricity. Hierarchical Bayesian inference over the population supports the triple formation channel.
The through-claim: eccentricity is a formation fingerprint. A circular orbit erases its history — the binary could have formed any number of ways. An eccentric orbit at merger frequency preserves information about the dynamical environment that produced it. The more eccentric the orbit, the more it remembers about its birth.