Observation of Gravitational Waves from the Coalescence of a $2.5\text{-}4.5~M

Observation of Gravitational Waves from the Coalescence of a $2.5\text{-}4.5~M_\odot$ Compact Object and a Neutron Star

Observation of Gravitational Waves from the Coalescence of a $2.5\text{-}4.5~M_\odot$ Compact Object and a Neutron Star ■

LIGO Scientific Collaboration, Alexandre Sevrin, Virgo Collaboration, Alba Romero-Rodríguez, KAGRA Collaboration, Hannah Duval, Kevin Turbang, Alba Romero-Rodríguez, Hannah Duval, Alexandre Sevrin, Kevin Turbang

Abstract ■

We report the observation of a coalescing compact binary with component masses \$2.5\textbackslash{}text\{-\}4.5\textasciitilde{}M\_\textbackslash{}odot\$ and \$1.2\textbackslash{}text\{-\}2.0\textasciitilde{}M\_\textbackslash{}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\textasciitilde{}M\_\textbackslash{}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 provisionally estimate a merger rate density of \$55\textasciicircum{}\{+127\}\_\{-47\}\textasciitilde{}\textbackslash{}text\{Gpc\}\textasciicircum{}\{-3\}\textbackslash{},\textbackslash{}text\{yr\}\textasciicircum{}\{-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.