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

TL;DR

This paper investigates the formation and detection of eccentric neutron star-black hole mergers, highlighting the potential of field triples to produce such systems and assessing their observability with current gravitational-wave searches.

Contribution

It introduces a method to estimate the minimum measurable eccentricity of NSBH mergers and evaluates the impact of eccentricity on detection efficiency, emphasizing the role of isolated field triples.

Findings

Eccentricity at 10 Hz can be as low as 0.003 for certain systems.

Current searches recover only 46% of eccentric NSBH mergers.

A significant fraction of NSBH detections could originate from field triples.

Abstract

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.