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Abstract

In August 2017, the first detection of a binary neutron star merger,
GW170817, made it possible to study neutron stars in compact binary systems
using gravitational waves. Despite being the loudest (in terms of
signal-to-noise ratio) gravitational wave detected to date, it was not possible
to unequivocally determine that GW170817 was caused by the merger of two
neutron stars instead of two black holes from the gravitational-wave data
alone. That distinction was largely due to the accompanying electromagnetic
counterpart. This raises the question: under what circumstances can
gravitational-wave data alone, in the absence of an electromagnetic signal, be
used to distinguish between different types of mergers? Here, we study whether
a neutron-star--black-hole binary merger can be distinguished from a binary
black hole merger using gravitational-wave data alone. We build on earlier
results using chiral effective field theory to explore whether the data from
LIGO and Virgo, LIGO A+, LIGO Voyager, or Cosmic Explorer could lead to such a
distinction. The results suggest that the present LIGO-Virgo detector network
will most likely be unable to distinguish between these systems even with the
planned near-term upgrades. However, given an event with favorable parameters,
third-generation instruments such as Cosmic Explorer will be capable of making
this distinction. This result further strengthens the science case for
third-generation detectors.