Distinguishing Neutron Star vs. Low-Mass Black Hole Binaries with Postmerger Gravitational Waves $-$ Sensitivity to Transmuted Black Holes and Non-Annihilating Dark Matter

Abstract: The astrophysical origin of observed low-mass compact binary coalescences in the 1-2.5 $M_{\odot}$ range remains ambiguous. Both binary neutron star (BNS) and binary low-mass black hole (LMBH) mergers produce nearly identical inspiral waveforms, and electromagnetic follow-up is not always possible. Distinguishing between these scenarios therefore presents a key challenge. We demonstrate that waveform differences in the late-inspiral to postmerger epochs create significant mismatches that will be detectable by planned detectors, viz., NEMO, Cosmic Explorer, and Einstein Telescope, while the currently operational LIGO A+ will be effective only for nearby sources. These differences are enhanced for stiffer equations of state. We show how the redshift-dependent compact binary merger rate inferred from gravitational wave observations can be parsed into BNS and LMBH components, accounting for misclassification probability. We forecast model-independent 90% exclusion sensitivities for the LMBH fraction. Interpreting these LMBHs as dark matter capture-induced transmuted black holes, we convert exclusion sensitivities into projected exclusion bounds on heavy non-annihilating dark matter. Our results illustrate how gravitational wave measurements can disentangle compact object populations and provide new insights into particle dark matter interactions.