Astrophysical Implications of Eccentricity in Gravitational Waves from Neutron Star-Black Hole Binaries

The gravitational-wave signal from the neutron star-black hole (NSBH) merger GW200105 is consistent with this binary having significant orbital eccentricity close to merger. This raises the question of how eccentric NSBHs form. Compact binaries that evolve in isolation radiate away any orbital eccentricity long before their gravitational-wave signal enters the sensitive frequency range of the LIGO-Virgo-KAGRA detector network. Meanwhile, dynamical environments -- which can be conducive to mergers on eccentric orbits -- produce very few NSBHs. Here, we focus on a formation channel that efficiently produces NSBHs with both misaligned spins and significant eccentricity close to merger: isolated field triples. We estimate the minimum measurable eccentricity of NSBHs at 10 Hz orbit-averaged gravitational-wave frequency, e_min,10, finding that for GW200105, GW200115, and GW230529-like systems, e_min,10 is O(0.01). For a GW190814-like unequal-mass binary with significant higher-order mode content, e_min,10 = 0.003; this is an order of magnitude lower than when higher modes are not present. For dominant-mode signals from binaries with $m_2=1.5$~M$_\odot$ and total masses from $3\,{\rm M_\odot} \leq M \leq50\,\rm M_\odot$, we find $0.008 \leq$ e_min,10 $\leq 0.022$. The relationship between $M$ and e_min,10 is linear when the binaries are non-spinning. When the binaries are maximally spin-precessing, e_min,10 decreases as mass ratio becomes more unequal. We estimate the sensitivity of a quasi-circular aligned-spin templated search to NSBH mergers from field triples, finding that we recover only 46% of systems that would have been detected with a search containing the full physics of the injected population. Finally, we show that if $\sim\nicefrac{1}{3}$ of present NSBH detections are measurably eccentric, then $\geq40\%$ are consistent with an isolated field triple origin.