Observation of Gravitational Waves from the Coalescence of a 2.5-4.5 M Compact Object and a Neutron Star

We report the observation of a coalescing compact binary with component masses 2.5-4.5 M and 1.2-2.0 M (all measurements quoted at the 90% credible level). The gravitational-wave signal GW230529 181500 was observed during the fourth observing run of the LIGO-Virgo-KAGRA detector network on 2023 May 29 by the LIGO Livingston Observatory. The primary component of the source has a mass less than 5 M at 99% credibility. We cannot definitively determine from gravitational-wave data alone whether either component of the source is a neutron star or a black hole. However, given existing estimates of the maximum neutron star mass, we find the most probable interpretation of the source to be the coalescence of a neutron star with a black hole that has a mass between the most massive neutron stars and the least massive black holes observed in the Galaxy. We provisionally estimate a merger rate density of 55+127-47 Gpc-3, yr-1 for compact binary coalescences with properties similar to the source of GW230529 181500; assuming that the source is a neutron star-black hole merger, GW230529 181500-like sources constitute about 60% of the total merger rate inferred for neutron star-black hole coalescences. The discovery of this system implies an increase in the expected rate of neutron star-black hole mergers with electromagnetic counterparts and provides further evidence for compact objects existing within the purported lower mass gap.