Astrophysical Implications of Eccentricity in Gravitational Waves from Neutron Star-Black Hole Binaries
Authors
Isobel Romero-Shaw
Jakob Stegmann
Gonzalo Morras
Andris Dorozsmai
Michael Zevin
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 an eccentric NSBH might form. Compact object binaries that form via isolated binary star evolution should 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. We estimate the minimum measurable eccentricity of NSBHs at 10 Hz orbit-averaged gravitational-wave frequency, $e_{\mathrm{min},10}$, finding that for GW200105, GW200115, and GW230529-like systems, $e_{\mathrm{min},10}$ is O(0.01). For a GW190814-like unequal-mass binary with significant higher-order mode content, $e_{\mathrm{min},10}=0.003$; this is an order of magnitude lower than when higher-order modes are not present. For dominant-mode signals from eccentric binaries with $m_2=1.5$ M$_\odot$ and a range of total masses from $3\,{\rm M_\odot} \leq M \leq50\,\rm M_\odot$, we find $0.008\leq e_{\mathrm{min},10}\leq0.022$. The relationship between $M$ and $e_{\mathrm{min},10}$ is linear when the binaries are non-spinning. When the binaries are maximally spin-precessing, $e_{\mathrm{min},10}$ decreases as mass ratio becomes more unequal. We estimate the sensitivity of a quasi-circular templated search to a population of NSBHs from field triples, finding that we recover only 27% of our simulated population. Finally, we show that if ~1/3 of present NSBH detections are measurably eccentric, then all of them are consistent with an isolated field triple origin.
