Characterizing Compact Object Binaries in the Lower Mass Gap with Gravitational Waves

Abstract: The gravitational wave event GW230529 was the first compact binary merger observation reported in the fourth observing run of the LIGO--Virgo--KAGRA observatory network. The more massive component of the GW230529 source binary was inferred to contain a compact object in the lower mass gap, a purported gap between the most massive neutron stars and least massive black holes based on compact object observations in the Milky Way. However, the properties of the mass-gap object, such as its exact mass and spin, remain unresolved due to statistical uncertainties. To investigate the properties of the GW230529 source binary, we perform parameter estimation on a suite of simulated gravitational wave systems with similar parameters. We vary the intrinsic properties of the simulated systems, the detector noise properties, the signal-to-noise ratios, and the waveform model used in recovery. We find that the low signal-to-noise ratio of GW230529 is the key reason for the ambiguity in determining whether the mass of the primary object in the binary is consistent with a low-mass black hole or a high-mass neutron star, since the priors for the masses and spins have a significant impact on the posterior distribution. The inclusion of tidal effects in the waveform model also contributes to the observed degeneracies in the posteriors. We show that a higher signal-to-noise ratio in future observations of mass-gap gravitational wave sources will increase the precision of the measurements and allow us to determine whether the components are black holes or neutron stars.