Observation of Gravitational Waves from the Coalescence of a $2.5\text{-}4.5~M_\odot$ Compact Object and a Neutron Star (2404.04248v3)

Abstract: We report the observation of a coalescing compact binary with component masses $2.5\text{-}4.5~M_\odot$ and $1.2\text{-}2.0~M_\odot$ (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 provisionally estimate a merger rate density of $55^{+127}_{-47}~\text{Gpc}^{-3}\,\text{yr}^{-1}$ 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.

• The paper reports the detection of gravitational waves from a merger between a 2.5–4.5 M☉ compact object and a neutron star, questioning the conventional lower mass gap.
• It employs independent matched-filter pipelines and Bayesian inference to constrain component masses and estimate a merger rate density of about 55 Gpc⁻³yr⁻¹.
• The study highlights potential electromagnetic counterparts in such mergers, underscoring the importance of multimessenger observations in astrophysics.

Analysis of Gravitational-Wave Detection from Compact Objects

The paper presents an analysis of the gravitational-wave (GW) signal GW230529_181500, observed at the LIGO Livingston observatory, involving a merger of compact objects. The paper was a collaborative effort among the LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration. The central focus is the characterized merger of a compact object with masses in the range of $2.5\text{--}4.5~M_\odot$ and a neutron star. This event stands out due to the mass of one component being situated in the lower mass gap between neutron stars and black holes commonly observed in our galaxy.

Key Results

• Component Masses and Characteristics: The analysis constrains the primary component mass to $< 5~M_\odot$ at $#1{GW230529ay_combined_imrphm_high_spin}\%$ credibility and indicates that the components might be a neutron star and a black hole. Despite the inability to definitively ascertain whether one or both components are black holes, the mass range aligns with neutron star-black hole (NSBH) binaries.
• Merger Rate Estimation: The inferred merger rate density for systems resembling GW230529_181500 is approximately 55 Gpc$^{-3}$yr$^{-1}$; these systems comprise around 60% of the total NSBH merger rate as established by the analysis' framework.
• Astrophysical Implications: The paper addresses the existence of compact objects in the lower mass gap, suggesting these could occur more frequently than previously understood based on Galactic observations alone. The detection of GW230529 provides additional evidence supporting this hypothesis.
• Prospects for Electromagnetic Counterparts: Due to the asymmetric mass distribution of the components, if this event were an NSBH merger, the likelihood of having electromagnetic counterparts increases relative to other observed NSBH systems.

Methodology and Analysis

The detection was managed using three independent matched-filter search pipelines—GstLAL, MBTA, and PyCBC—each contributing to the significance estimation of the observed signal. Detailed parameter estimation was conducted using Bayesian inference techniques with waveform models, such as \emph{IMRPhenomXPHM} and \emph{SEOBNRv5PHM}, encapsulating various physical effects including precession and higher-order multipoles. Comparisons between models explored the impacts of tidal deformability and other effects, although little tidal information was recovered from the signal due to moderate signal-to-noise ratios.

Observations and Theoretical Implications

1. Existence of a Lower Mass Gap: The detection significantly challenges the conventional understanding of a deserted mass gap between neutron stars and black holes, motivating the revision of theoretical models regarding stellar evolution and binary mergers.
2. Spin and Mass Ratio Distributions: The spin and mass ratio posterior distributions illuminate possible configurations consistent with the observed data, offering detailed insights into the physics of compact object mergers.
3. Multimessenger Astronomy: While no electromagnetic counterpart was detected for this event due to the inadequate localization by a single observatory, the potential for future similar events to be multimessenger sources is enhanced by the probability of such configurations.

Future Directions

This paper strengthens the rationale for continued observations in gravitational-wave astronomy. The findings underscore the potential for complementary data from electromagnetic observations to contribute richly to our understanding of such complex cosmic phenomena. The results also suggest that future advancements in GW detection sensitivity and analysis techniques will likely yield further discoveries with profound implications for astrophysics, notably in characterizing objects within the lower mass gap and refining our understanding of compact binary coalescences.