[Paper Review] Nuclear Physics Multimessenger Astrophysics Constraints on the Neutron Star Equation of State: Adding NICER's PSR J0740+6620 Measurement

P. T. H. Pang, Ingo Tews+4 more|arXiv (Cornell University)|Jan 1, 2021

Pulsars and Gravitational Waves ResearchPhysics and Astronomy100 references137 citations

TL;DR

This paper revisits constraints on the neutron star equation of state using a multi-messenger framework that combines NICER's new radius measurement of the massive pulsar PSR J0740+6620 with gravitational wave data (GW170817, GW190425), kilonova and gamma-ray burst observations, and nuclear theory. The updated analysis yields a 1.4 solar mass neutron star radius of 11.94+0.76−0.87 km at 90% confidence and disfavors a strong first-order phase transition to quark matter.

ABSTRACT

In the past few years, new observations of neutron stars (NSs) and NS mergers have provided a wealth of data that allow one to constrain the equation of state (EOS) of nuclear matter at densities above nuclear saturation density. However, most observations were based on NSs with masses of about 1.4 M⊙, probing densities up to ∼three to four times the nuclear saturation density. Even higher densities are probed inside massive NSs such as PSR J0740+6620. Very recently, new radio observations provided an update to the mass estimate for PSR J0740+6620, and X-ray observations by the NICER and XMM telescopes constrained its radius. Based on these new measurements, we revisit our previous nuclear physics multimessenger astrophysics constraints and derive updated constraints on the EOS describing the NS interior. By combining astrophysical observations of two radio pulsars, two NICER measurements, the two gravitational-wave detections GW170817 and GW190425, detailed modeling of the kilonova AT 2017gfo, and the gamma-ray burst GRB 170817A, we are able to estimate the radius of a typical 1.4 M⊙ NS to be 11.94-0.87+0.76 km at 90% confidence. Our analysis allows us to revisit the upper bound on the maximum mass of NSs and disfavors the presence of a strong first-order phase transition from nuclear matter to exotic forms of matter, such as quark matter, inside NSs.

Motivation & Objective

• To update constraints on the neutron star equation of state using the latest multi-messenger observations.
• To assess the impact of NICER's new radius measurement of PSR J0740+6620 on the equation of state and phase transition likelihood.
• To refine estimates of the maximum neutron star mass and the probability of GW190814 being a neutron star-black hole merger.
• To evaluate whether the high-density EOS probed by PSR J0740+6620 supports a first-order phase transition to exotic matter.

Proposed method

• A nuclear-physics multi-messenger astrophysics (NMMA) framework is employed, integrating observational data with theoretical modeling of the equation of state.
• The analysis combines gravitational wave signals from GW170817 and GW190425 with electromagnetic counterparts, including kilonova AT2017gfo and gamma-ray burst GRB170817A.
• NICER's X-ray pulse profile modeling of PSR J0740+6620 provides a mass-radius constraint, with two independent posterior samples from Riley et al. (2021b) and Miller et al. (2021b).
• XMM-Newton X-ray data is included to improve radius constraints, and Bayesian model comparison is used to assess phase transition likelihood.
• The framework incorporates chiral effective field theory at low densities and uses posterior predictive checks to validate model consistency.
• Bayes factors are computed to evaluate the probability of a first-order phase transition versus a smooth equation of state.

Experimental results

Research questions

• RQ1 How does the inclusion of NICER's PSR J0740+6620 radius measurement refine the neutron star equation of state at supra-nuclear densities?
• RQ2 What is the updated 90% credible interval for the radius of a 1.4 solar mass neutron star?
• RQ3 Does the high-mass, high-density data from PSR J0740+6620 support a first-order phase transition to quark matter?
• RQ4 How does the new data affect the probability that GW190814 originated from a neutron star-black hole merger?
• RQ5 To what extent do the NICER and XMM-Newton data improve constraints on the maximum neutron star mass?

Key findings

• The radius of a 1.4 solar mass neutron star is constrained to 11.94+0.76−0.87 km at 90% credible interval, showing excellent agreement with prior predictions.
• The inclusion of NICER's PSR J0740+6620 measurement results in only small shifts in the radius estimate, due to the 10–20% uncertainty in the radius measurement.
• The Bayes factor for a first-order phase transition is 0.29 ± 0.01 (0.23 ± 0.01 with XMM data), indicating disfavor of such a transition, though not ruled out.
• The maximum neutron star mass is constrained to 2.15+0.15−0.12 M⊙, with no significant change from previous estimates.
• The probability that GW190814 is a neutron star-black hole merger drops from 19% to 6.3–15.2% with the inclusion of NICER and NICER+XMM data, supporting a black hole origin.
• The NICER data alone shows good agreement with prior NMMA predictions, validating the framework, but the XMM data slightly favors a stiffer equation of state, though the effect is small due to measurement uncertainties.