Constraints on the dense matter equation of state and neutron star properties from NICER's mass-radius estimate of PSR J0740+6620 and multimessenger observations (2105.06981v2)

Abstract: In recent years our understanding of the dense matter equation of state (EOS) of neutron stars has significantly improved by analyzing multimessenger data from radio/X-ray pulsars, gravitational wave events, and from nuclear physics constraints. Here we study the additional impact on the EOS from the jointly estimated mass and radius of PSR J0740+6620, presented in Riley et al. (2021) by analyzing a combined dataset from X-ray telescopes NICER and XMM-Newton. We employ two different high-density EOS parameterizations: a piecewise-polytropic (PP) model and a model based on the speed of sound in a neutron star (CS). At nuclear densities these are connected to microscopic calculations of neutron matter based on chiral effective field theory interactions. In addition to the new NICER data for this heavy neutron star, we separately study constraints from the radio timing mass measurement of PSR J0740+6620, the gravitational wave events of binary neutron stars GW190425 and GW170817, and for the latter the associated kilonova AT2017gfo. By combining all these, and the NICER mass-radius estimate of PSR J0030+0451 we find the radius of a 1.4 solar mass neutron star to be constrained to the 95% credible ranges 12.33^{+0.76}_{-0.81} km (PP model) and 12.18^{+0.56}_{-0.79} km (CS model). In addition, we explore different chiral effective field theory calculations and show that the new NICER results provide tight constraints for the pressure of neutron star matter at around twice saturation density, which shows the power of these observations to constrain dense matter interactions at intermediate densities.

• The paper advances dense matter EOS constraints by integrating NICER X-ray measurements with gravitational wave observations.
• It compares piecewise-polytropic and speed-of-sound models, resulting in neutron star radius estimates around 12 km from PSR J0740+6620.
• The multimessenger analysis refines nuclear physics parameters and enhances understanding of neutron star interiors for future research.

Overview of Neutron Star Properties from \NICER and Multimessenger Observations

The research paper by Raaijmakers et al. presents significant advancements in understanding the dense matter equation of state (EOS) of neutron stars by integrating diverse observational data, including those from \NICER and gravitational wave detectors. Neutron stars, which are dense remnants from supernovae, offer crucial insights into nuclear physics under extreme conditions, and estimating their mass and radius is critical in constraining their EOS.

Methodological Approach

The paper evaluates the EOS using a comprehensive set of observations, notably the constraints derived from the \NICER X-ray measurements of the pulsars PSR J0740+6620 and PSR J0030+0451. The paper incorporates data from gravitational wave events such as GW170817 and GW190425, and considerations of associated electromagnetic counterparts, like the kilonova AT2017gfo.

Raaijmakers et al. employ two EOS parameterizations: a piecewise-polytropic (PP) model and a model based on the speed of sound (CS), integrating constraints from chiral effective field theory at low densities. This approach allows the researchers to explore the possible pressure and structural transitions within neutron stars which may bear implications for phenomena like phase transitions or the presence of exotic matter.

Results and Key Findings

The analysis constrains the neutron star radius effectively, leveraging the mass-radius estimates derived from \NICER and XMM-Newton. For example, using the data from PSR J0740+6620, the core EOS models indicated credible neutron star radii of $12.33 \pm 0.76$ km and $12.18 \pm 0.67$ km for the piecewise-polytropic and speed-of-sound models, respectively. These findings cohere with previous result ranges and are consistent regardless of different chiral EFT interactions used.

Additionally, the paper investigates the potential role of gravitational wave data in refining EOS predictions. The tidal deformability constraints obtained from GW170817 further guide the pressure characteristics at multiple-density scales, enabling more precise estimates of EOS stiffness.

Implications for Future Research

These findings have marked implications for both theoretical and applied physics domains. Practically, the tight constraints on neutron star radii and the identification of plausible EOS scenarios aid in better understanding the physics underpinning these dense celestial bodies. Theoretical implications extend to the potential validation or refutation of exotic matter presence and the nature of phase transitions within neutron star interiors.

Future developments in observational capabilities, particularly with the upcoming advancements in X-ray and gravitational wave observatories, promise further refinements in such studies. The continuum of data accumulation aims to achieve narrower constraints, fostering advancements in nuclear astrophysics and bridging theoretical and observational astrophysical research.

In conclusion, this work demonstrates a pivotal step forward in multi-messenger astrophysics, integrating data from various observational fronts to build a coherent understanding of neutron star interiors. This comprehensive approach exemplifies how sophisticated modeling combined with precise empirical work can significantly enhance our knowledge of some of the universe's densest objects.