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 _ { \mathrm { max } } = 2.064 ^ { +0.260 } _ { -0.134 } ( 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 ^ { \mathrm { NS } } _ { \mathrm { 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 .