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Origin of the bimodal-like distribution in EOS posteriors

Investigate the origin of the bimodal-like distribution observed in the posterior distributions of the neutron-star equation of state inferred using the piecewise-polytropic and speed-of-sound parameterizations with NICER mass–radius measurements and gravitational-wave data, and ascertain whether this bimodality is caused by tension between the PSR J0740+6620 posteriors and those of PSR J0437−4715 and GW170817.

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Background

The paper performs Bayesian inference of the neutron-star equation of state (EOS) using updated NICER mass–radius measurements (PSR J0437−4715, PSR J0740+6620, PSR J0030+0451), gravitational-wave tidal deformabilities (GW170817, GW190425), and new chiral effective field theory constraints up to 1.5 times nuclear saturation density. Two EOS parameterizations are employed: a piecewise-polytropic (PP) model and a speed-of-sound (CS) model.

In analyzing posteriors, the authors observe a bimodal-like structure in EOS distributions. They suggest this may reflect tension between the high-radius preference from PSR J0740+6620 and the lower-radius indications from PSR J0437−4715 and GW170817, but emphasize that the precise origin of the bimodality is not yet established and requires further investigation.

References

Although our inferences suggest that this bimodal-like structure is due to a tension between the posteriors of J0740 and those of J0437 and GW170817, further investigations to better understand its origin are needed, which we leave for future work.

— Constraining the dense matter equation of state with new NICER mass-radius measurements and new chiral effective field theory inputs (2407.06790 - Rutherford et al., 9 Jul 2024) in Subsection “Systematic uncertainties” within Section “Discussion and conclusions”