Nonparametric Inference of Neutron Star Composition, Equation of State, and Maximum Mass with GW170817

The detection of GW170817 in gravitational waves provides unprecedented constraints on the equation of state (EOS) of the ultra-dense matter within the cores of neutron stars (NSs). We extend the nonparametric analysis first introduced in Landry & Essick (2019), and confirm that GW170817 favors soft EOSs. We infer macroscopic observables for a canonical 1.4 $M_{\odot}$ NS, including the tidal deformability $\Lambda_{1.4} = 211^{+312}_{-137}$ ($491^{+216}_{-181}$) and radius $R_{1.4}= 10.86^{+2.04}_{-1.42}$ ($12.51^{+1.00}_{-0.88}$) km, as well as the maximum mass for nonrotating NSs, $M_{max} = 2.064^{+0.260}_{-1.34}$ ($2.017^{0.238}_{-0.087}$) $M_\odot$, with nonparametric priors loosely (tightly) constrained to resemble candidate EOSs from the literature. Furthermore, we find weak evidence that GW170817 involved at least one NS based on gravitational-wave data alone ($B^{NS}_{BBH}= 3.3 \pm 1.4$), consistent with the observation of electromagnetic counterparts. We also investigate GW170817's implications for the maximum spin frequency of millisecond pulsars, and find that the fastest known pulsar is spinning at more than 50% of its breakup frequency at 90% confidence. We additionally find modest evidence in favor of quark matter within NSs, and GW170817 favors the presence of at least one disconnected hybrid star branch in the mass--radius relation over a single stable branch by a factor of 2. Assuming there are multiple stable branches, we find a suggestive posterior preference for a sharp softening around nuclear density followed by stiffening around twice nuclear density, consistent with a strong first-order phase transition. While the statistical evidence in favor of new physics within NS cores remains tenuous with GW170817 alone, these tantalizing hints reemphasize the promise of gravitational waves for constraining the supranuclear EOS.