Search

We investigate the neutron star (NS) equation of state (EOS) by incorporating multi-messenger data of GW170817, PSR J0030 + 0451, PSR J0740 + 6620, and state-of-the-art theoretical progresses, including the information from chiral effective field theory (χ EFT) and perturbative quantum chromodynamics (pQCD) calculation. Taking advantage of the various structures sampling by a single-layer feed-forward neural network model embedded in the Bayesian nonparametric inference, the structure of NS matter's sound speed c s is explored in a model-agnostic way. It is found that a peak structure is common in the c s 2 posterior, locating at (2.4 - 4.8) ρ sat (nuclear saturation density) and c s 2 exceeds c 2 / 3 at 90% credibility. The non-monotonic behavior suggests evidence of the state deviating from the hadronic matter inside the very massive NSs. Assuming the new/exotic state is featured as it is softer than typical hadronic models or even with hyperons, we find that a sizable (⩾ 10 - 3 M ⊙ ) exotic core, likely made of quark matter, is plausible for the NS with a gravitational mass above about 0.98 M TOV , where M TOV represents the maximum gravitational mass of a non-rotating cold NS. The inferred M TOV = (2.18 - 0.13 + 0.27) M ⊙ (90% credibility) is well consistent with the value of (2.17 - 0.12 + 0.15) M ⊙ estimated independently with GW170817/GRB 170817A/AT2017gfo assuming a temporary supramassive NS remnant formed after the merger. PSR J0740 + 6620, the most massive NS detected so far, may host a sizable exotic core with a probability of ≈ 0.36. [ABSTRACT FROM AUTHOR]