Challenging a binary neutron star merger interpretation of GW230529

TL;DR

GW230529 presents a low-SNR gravitational-wave signal whose primary mass sits in the proposed mass gap, raising the question of BHNS versus BNS origin. The authors generate GW230529-like NR simulations for both interpretations, hybridize them with phenomenological models, and perform Bayesian parameter estimation across waveform models and priors to compare with the actual data. They find the BHNS scenario is more consistent with GW230529, though a BNS origin with highly spinning NSs cannot be excluded, and they show that EM counterparts would be dim without disk winds but potentially detectable with wind ejecta by upcoming surveys. The work highlights waveform-degeneracy challenges, validates NR-informed hybrids, and informs follow-up strategies for similar low-SNR, high-mass binaries.

Abstract

GW230529_181500 represented the first gravitational-wave detection with one of the component objects' mass inferred to lie in the previously hypothesized mass gap between the heaviest neutron stars and the lightest observed black holes. Given the expected maximum mass values for neutron stars, this object was identified as a black hole, and, with the secondary component being a neutron star, the detection was classified as a neutron star-black hole merger. However, due to the low signal-to-noise ratio and the known waveform degeneracy between the spin and mass ratio in the employed gravitational-wave models, GW230529_181500 could also be interpreted as a merger of two heavy ($\gtrsim 2 \mathrm{M}_\odot$) neutron stars with high spins. We investigate the distinguishability of these scenarios by performing parameter estimation on simulated signals obtained from numerical-relativity waveforms for both neutron star-black hole and binary neutron star systems, with parameters consistent with GW230529_181500, and comparing them to the analysis of the real event data. We find that GW230529_181500 is more likely to have originated from a neutron star-black hole merger, though the possibility of a binary neutron star origin can not be ruled out. Moreover, we use the simulation data to estimate the signatures of potential electromagnetic counterparts emitted by the systems. We find them to be too dim to be located by current wide-field surveys if only the dynamical ejecta is considered, and detectable by the Vera C. Rubin Observatory during the first two days after merger if one accounts for additional disk wind ejecta.

Figures (9)

• Figure 1: Top panels: Real part of the (2,2)-mode of the spin-weighted spherical harmonic coefficients as a function of retarded time before merger with the extraction radius $r=1200$ scaled out. The merger time reference is taken to be that of the BHNS_DD2 configuration. The vertical solid lines show the corresponding merger times for each configuration. The vertical dashed gray lines indicate the alignment window. Bottom panel: Phase differences between each configuration with respect to the BHNS_DD2 one.
• Figure 2: Phase difference between the waveform models and the hybrids for all the configurations. The vertical dashed gray lines signify the alignment window.
• Figure 3: Posterior distributions for the mass ratio $q$ and effective spin $\chi_{\mathrm{eff}}$ for the analysis with IMRPhenomXAS_NRTidalv3 and the AS prior (left) and the DV prior (right) for the injections and for the GW230529 event data. The contours in the $q$-$\chi_{\mathrm{eff}}$ panel correspond to the 90% credible level. The orange vertical and horizontal lines mark the BHNS injected parameters, while the blue ones mark the BNS ones. The priors for both quantities are shown as gray lines. To aid the visual comparison, the prior probability densities for the effective spin are multiplied by four.
• Figure 4: Posterior distributions for $q$ and $\chi_{\mathrm{eff}}$ recovered with different waveform models using the AS prior (left two columns) and the DV prior (right two columns). The vertical lines mark the injected values for the BHNS and BNS cases. The priors for both quantities are plotted as grey lines. To aid the visual comparison, the prior probability densities for the effective spin are multiplied by four.
• Figure 5: Same as for Figure \ref{['figure:qchi_v4_aligned_spins']} but for precessing waveforms.