Characterizing Compact-object Binaries in the Lower Mass Gap with Gravitational Waves

Abstract The source binary of the gravitational-wave (GW) event GW230529, detected at the beginning of the fourth LIGO-Virgo-KAGRA observing run, was inferred to consist of a neutron star (NS) and a compact object in the lower mass gap, a purported gap between the most massive NSs (∼3 M ⊙ ) and least massive black holes (BHs; ∼5 M ⊙ ) based on compact-object observations in the Milky Way. While the nature of the mass-gap object could not be determined from the GW data alone for this event, definitively distinguishing whether this object is a NS or BH would have profound implications for the NS equation of state, supernova physics, and multimessenger astronomy. In this work, we perform parameter estimation on a suite of simulated GW systems with parameters similar to those of the GW230529 source binary to investigate whether the ambiguity in the physical nature of the source is a generic result for such systems. We vary the intrinsic properties of the simulated systems, the detector noise properties, the signal-to-noise ratios (SNRs), and the waveform model used in recovery. We find that the low SNR 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 BH or a high-mass NS, and thus the priors used for the masses and spins have a significant impact on the posterior distribution, which is a generic result for low-SNR events. The inclusion of tidal effects in the waveform model also contributes to the observed degeneracies in the posteriors, since the statistical uncertainties in analyses of GW events like GW230529 are larger for waveform models including tidal effects. We show that the future observation of such a system with a higher SNR (∼30) would increase the precision of the mass measurements sufficiently to allow us to determine the nature of the mass-gap object.