Distinguishing between Black Holes and Neutron Stars within a Population of Weak Tidal Measurements

We study the ability of tidal signatures within the inspiral of compact binaries observed through gravitational waves (GWs) to distinguish between neutron stars (NSs) and black holes (BHs). After quantifying how hard this measurement is on a single-event basis, we investigate the ability of a large catalog of GW detections to constrain the fraction of NS in the population as a function of mass: $f_{\mathrm{N}\mathrm{S}}(m)$. Using simulated catalogs with realistic measurement uncertainty, we find that $>O(200)$ events will be needed before we can precisely measure $f_{\mathrm{N}\mathrm{S}}$, and catalogs of $>O(100)$ events will be needed before we can even rule out the possibility that all low-mass objects are BHs with GW data alone (i.e., without electromagnetic counterparts). Therefore, this is unlikely to occur with advanced detectors, even at design sensitivity. Nevertheless, it could be feasible with next-generation facilities like Cosmic Explorer and Einstein Telescope.