Plausible presence of new state in neutron stars with masses above $0.98M_{\mathrm{TOV}}$

Abstract

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 ($\chi$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 $c_{\mathrm{s}}$ is explored in a model-agnostic way. It is found that a peak structure is common in the $c_{\mathrm{s}}^2$ posterior, locating at $2.4-4.8{\rho }_{\mathrm{sat}}$ (nuclear saturation density) and $c_{\mathrm{s}}^2$ exceeds $c^2/3$ at 90\% credibility. The non-monotonic behavior suggests evidence of the state deviating from 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 ($\ge 10^{-3}{M}_{\odot }$) exotic core, likely made of quark matter, is plausible for the NS with a gravitational mass above about $0.98M_{\mathrm{TOV}}$, where $M_{\mathrm{TOV}}$ represents the maximum gravitational mass of a non-rotating cold NS. The inferred $M_{\mathrm{TOV}}=2.18_{-0.13}^{+0.27}{M}_{\odot }$ (90\% credibility) is well consistent with the value of $2.17_{-0.12}^{+0.15}{M}_{\odot }$ 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 an exotic core with a probability of $\approx 0.36$.

8+5 pages, 6+5 figures, published in Science Bulletin

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