Orbital eccentricity in a neutron star - black hole binary merger

TL;DR

This study reports the first robust evidence of orbital eccentricity in a neutron star–black hole binary, GW200105, by employing a novel precession+eccentric waveform (pyEFPEM) within a Bayesian inference framework. The analysis yields e_{20} = 0.145^{+0.007}_{-0.063} at 20 Hz, with a 99% lower limit of 0.028, and indicates a low, possibly negligible, spin for the components, together with a highly unequal mass ratio (q ≈ 0.13). These findings challenge the isolated-binary evolution scenario and favor dynamical formation channels, such as hierarchical triples or dense stellar environments. The work demonstrates the necessity of incorporating eccentricity in GW analyses to avoid biases in mass and spin inferences, and sets the stage for future population studies of eccentric NSBH binaries with next-generation detectors.

Abstract

The observation of gravitational waves 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 gravitational-wave signal that can be extracted from observations. To date, neither precession nor eccentricity have 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 gravitational-wave event GW200105 (R. Abbott et al. 2021a) and infer a median orbital eccentricity of $e_{20}\sim 0.145$ at an orbital period of 0.1s, ruling out eccentricities smaller than 0.028 with 99.5% confidence. We find inconclusive evidence for the presence of precession, consistent with previous, non-eccentric 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.

Figures (12)

• Figure 1: Measured eccentricity distribution in GW200105. One-dimensional posterior probability distribution for our measurement of the orbital eccentricity at a GW frequency of $20$ Hz. The solid vertical line indicates the median and the two dashed lines the 90% credible interval.
• Figure 2: Inferred eccentricity and precessing spin distributions in GW200105. One-dimensional and two-dimensional posterior probability distributions for our measurement of the orbital eccentricity and the precession spin $\chi_p$Schmidt:2014iyl at a GW frequency of $20$ Hz. The solid lines indicate the median, and the dashed lines the 90% credible interval. We find no correlation between precession and eccentricity for GW200105.
• Figure 3: Estimated eccentricity evolution of GW200105. The estimated orbital eccentricity of GW200105 as a function of the GW frequency Moore:2016qxz between $1$ mHz and $400$ Hz and the corresponding orbital period in seconds (top axis), along with the $90\%$ credible interval obtained from our measurement, assuming that the evolution is driven entirely by GW emission. The arrows indicate the sensitive regions of Advanced LIGO ($f_{\rm GW} \geq 20$Hz) and LISA ($f_{\rm GW} \leq 1$Hz).
• Figure 4: Eccentric-precessing analysis of BNS and NSBH events. One-dimensional posterior distributions for the eccentricity at 20 Hz inferred from Bayesian inference with the eccentric-precessing pyEFPE model and uniform eccentricity priors on the two BNS events, GW170817 and GW190425, and the NSBH events GW190426, GW190917, GW200115, GW200105 and GW230529. We find that, with the exception of GW200105, all other analysed events are highly consistent with circular binaries.
• Figure 5: Measured properties of GW200105. One- and two-dimensional posterior probability distributions of the component masses, effective inspiral spin $\chi_{\rm eff}$, effective precession spin $\chi_p$, and the orbital eccentricity at a GW frequency of $20$ Hz inferred with the eccentric-precessing pyEFPE waveform model. Solid lines indicate the median value, dashed lines the $90\%$ credible interval. Two-dimensional contours indicate the $50\%$ (solid) and $90\%$ (dashed) credible regions.