Discriminating same-mass neutron stars and black holes gravitational waveforms

Gravitational waveforms from coalescences of binary black hole and binary neutron star systems with low tidal effects can hardly be distinguished if the two systems have similar masses. In the absence of discriminating power based on the gravitational waveforms, the classification of sources into binary neutron stars, binary black holes and mixed systems can only be unambiguous when assuming the standard model of stellar evolution and using the fact that there exists a mass gap between neutron stars and black holes. This approach is however limited by its own assumptions: for instance the 2.6 solar mass object detected in the GW190814 event remains unclassified, and models of new physics can introduce new compact objects, like primordial black holes, which may have masses in the same range as neutron stars. In what follows, we investigate the possibility of discriminating between gravitational-wave signals emitted by different systems. First, we study the match between two waveforms, assuming several sensitivities of the detectors. In a second step, the ability of distinguishing one signal from the other is evaluated on simulations: a gravitational-wave signal is added to realistic noise from the LIGO-Virgo detectors, and the model best describing the simulated data is chosen based on the Bayes factor. The results depend strongly on the considered parameters, as the masses of the objects and tidal deformabilities of the neutrons stars. The task of distinguishing the nature of a compact object based on the gravitational-wave signal appears challenging for the current interferometers network. For instance, for a BNS system with tidal deformabilities <math display="inline"><mi mathvariant="normal">Λ</mi><mo>=</mo><mn>600</mn></math> and chirp mass <math display="inline"><mrow><mn>1.44</mn><mtext> </mtext><mtext> </mtext><msub><mrow><mi>M</mi></mrow><mrow><mo stretchy="false">⊙</mo></mrow></msub></mrow></math> and under optimistic assumptions, the nature of compact objects is correctly determined only for distances smaller that 150 Mpc, while it is unambiguously determined even for a distance of 300 Mpc in the case of third-generation detectors.