Does Matter Matter? Using the Mass Distribution to Distinguish Neutron Stars and Black Holes
Abstract
Gravitationalwave 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 gravitationalwave catalog, GWTC1, we investigate whether the powerlaw 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 powerlaw 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 powerlaw fit across the entire mass range is in mild tension with (a) the detection of one source in the BNS mass range (∼12.5 M_{⊙}), (b) the absence of detections in the "massgap" range (∼2.55 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 gravitationalwave data alone already suggest two distinct populations of compact objects.
 Publication:

The Astrophysical Journal
 Pub Date:
 August 2020
 DOI:
 10.3847/20418213/aba7b6
 arXiv:
 arXiv:2006.13178
 Bibcode:
 2020ApJ...899L...8F
 Keywords:

 Astrophysical black holes;
 Compact objects;
 Stellar mass black holes;
 Neutron stars;
 Gravitational wave astronomy;
 Gravitational waves;
 Astrostatistics;
 98;
 288;
 1611;
 1108;
 675;
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 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology
 EPrint:
 16 pages, including 2 page appendix, 7 figures. Updated to match published version