Does Matter Matter? Using the Mass Distribution to Distinguish Neutron Stars and Black Holes

Gravitational-wave detectors have opened a new window through which we can observe black holes (BHs) and neutron stars (NSs). Analyzing the 11 detections from LIGO/Virgo's first gravitational-wave catalog, GWTC-1, we investigate whether the power-law fit to the BH mass spectrum can also accommodate the binary neutron star (BNS) event GW170817, or whether we require an additional feature, such as a mass gap in between the NS and BH populations. We find that with respect to the power-law fit to binary black hole (BBH) masses, GW170817 is an outlier at the 0.13% level, suggesting a distinction between NS and BH masses. A single power-law fit across the entire mass range is in mild tension with (a) the detection of one source in the BNS mass range (∼1–2.5 M⊙), (b) the absence of detections in the "mass-gap" range (∼2.5–5 M⊙), and (c) the detection of 10 sources in the BBH mass range (≳5 M⊙). Instead, the data favor models with a feature between NS and BH masses, including a mass gap (Bayes factor of 4.6) and a break in the power law, with a steeper slope at NS masses compared to BH masses (91% credibility). We estimate the merger rates of compact binaries based on our fit to the global mass distribution, finding ${{R}}_{{\rm{B}}{\rm{N}}{\rm{S}}}={871}_{-805}^{+3015}$ and ${{ \mathcal R }}_{\mathrm{BBH}}={47.5}_{-28.8}^{+57.9}\ {\mathrm{Gpc}}^{-3}\ {\mathrm{yr}}^{-1}$. We conclude that, even in the absence of any prior knowledge of the difference between NSs and BHs, the gravitational-wave data alone already suggest two distinct populations of compact objects.