LIGO Document P2300352-v9

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

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We report the observation of a coalescing compact binary with component masses \( 2.5-4.5~M_\odot \) and \( 1.2-2.0~M_\odot \) (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 \( 5~M_\odot \) 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 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.
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