Item

Abstract

Two binary neutron star mergers, GW170817 and GW190425, have been detected by
Advanced LIGO and Virgo. These signals were detected by matched-filter searches
that assume the star's orbit has circularized by the time their
gravitational-wave emission is observable. This suggests that their
eccentricity is low, but a direct measurement of their eccentricity has not yet
been made. We use gravitational-wave observations to measure the eccentricity
of GW170817 and GW190425. We find that the eccentricity at a gravitational-wave
frequency of 10 Hz is $e \leq 0.024$ and $e \leq 0.048$ for GW170817 and
GW190425, respectively (90\% confidence). This is consistent with the binaries
being formed in the field, as such systems are expected to have circularized to
$e \leq 10^{-4}$ by the time they reach the LIGO-Virgo band. Our constraint is
a factor of two smaller that an estimate based on GW170817 being detected by
searches that neglect eccentricity. We note that other techniques used to
constrain binary neutron star eccentricity without full parameter estimation
may miss degeneracies in the waveform, and that for future signals it will be
important to perform full parameter estimation with accurate waveform
templates.