Search for eccentric NSBH and BNS mergers in the third observing run of Advanced LIGO and Virgo

The possible formation histories of neutron star binaries remain unresolved by current gravitational-wave catalogs. The detection of an eccentric binary system could be vital in constraining compact binary formation models. We present the first search for aligned spin eccentric neutron star-black hole binaries (NSBH) and the most sensitive search for aligned-spin eccentric binary neutron star (BNS) systems using data from the third observing run of the advanced LIGO and advanced Virgo detectors. No new statistically significant candidates are found; we constrain the local merger rate for specific astrophysical models to be less than 150 Gpc-3 yr-1 for binary neutron stars in the field, and, 50, 100, and 70 Gpc-3 yr-1 for neutron star-black hole binaries in globular clusters, hierarchical triples and nuclear clusters, respectively, at the 90% confidence level if we assume that no sources have been observed from these populations. We predict the capabilities of upcoming and next-generation observatory networks; we investigate the ability of three LIGO (A#) detectors and Cosmic Explorer CE (20 km)+CE (40 km) to use eccentric binary observations for determining the formation history of neutron star binaries. We find that 2-100 years of observation with three A# observatories are required before we observe clearly eccentric NSBH binaries; this reduces to only 10 days-1 year with the CE detector network. CE will observe tens to hundreds of measurably eccentric binaries from each of the formation models we consider.