Constraining the dense matter equation of state with new NICER mass-radius measurements and new chiral effective field theory inputs (2407.06790v2)

Abstract: Pulse profile modeling of X-ray data from NICER is now enabling precision inference of neutron star mass and radius. Combined with nuclear physics constraints from chiral effective field theory ($\chi$EFT), and masses and tidal deformabilities inferred from gravitational wave detections of binary neutron star mergers, this has lead to a steady improvement in our understanding of the dense matter equation of state (EOS). Here we consider the impact of several new results: the radius measurement for the 1.42$\,M_\odot$ pulsar PSR J0437$-$4715 presented by Choudhury et al. (2024), updates to the masses and radii of PSR J0740$+$6620 and PSR J0030$+$0451, and new $\chi$EFT results for neutron star matter up to 1.5 times nuclear saturation density. Using two different high-density EOS extensions -- a piecewise-polytropic (PP) model and a model based on the speed of sound in a neutron star (CS) -- we find the radius of a 1.4$\,M_\odot$ (2.0$\,M_\odot$) neutron star to be constrained to the 95% credible ranges $12.28^{+0.50}_{-0.76}\,$km ($12.33^{+0.70}_{-1.34}\,$km) for the PP model and $12.01^{+0.56}_{-0.75}\,$km ($11.55^{+0.94}_{-1.09}\,$km) for the CS model. The maximum neutron star mass is predicted to be $2.15^{+0.14}{-0.16}\,$$M\odot$ and $2.08^{+0.28}{-0.16}\,$$M\odot$ for the PP and CS model, respectively. We explore the sensitivity of our results to different orders and different densities up to which $\chi$EFT is used, and show how the astrophysical observations provide constraints for the pressure at intermediate densities. Moreover, we investigate the difference $R_{2.0} - R_{1.4}$ of the radius of 2$\,M_\odot$ and 1.4$\,M_\odot$ neutron stars within our EOS inference.

• The paper constrains the dense matter EOS using NICER mass-radius measurements and Bayesian inference, yielding precise radius estimates for 1.4 solar-mass neutron stars.
• It integrates multimessenger data, including gravitational wave detections and chiral EFT inputs, to enhance low-density EOS constraints.
• The study utilizes both piecewise-polytropic and speed-of-sound models to limit the maximum neutron star mass in agreement with high-precision pulsar observations.

Constraining the Dense Matter Equation of State with New Observations and Theoretical Inputs

The paper explores the constraints on the equation of state (EOS) for dense matter, specifically within the context of neutron stars (NSs). This work leverages new observational data from the Neutron Star Interior Composition Explorer (NICER) and theoretical results from chiral effective field theory (χEFT) to refine our understanding of the dense matter EOS.

Observational Data and Theoretical Developments

The analysis incorporates multiple sets of measurements:

1. NICER Mass-Radius Measurements: New data provides mass and radius estimates for key neutron stars such as PSR J0437-4715 (hereafter J0437), PSR J0740+6620 (J0740), and PSR J0030+0451 (J0030). This data, notably the precise radius constraints for J0437, are crucial in resolving uncertainties in the EOS, particularly around the nuclear saturation density.
2. Gravitational Wave Observations: Data from binary neutron star mergers detected via gravitational waves (GW170817 and GW190425) contribute constraints on tidal deformabilities, enriching the EOS inference through a multimessenger approach.
3. Chiral Effective Field Theory Inputs: Recent developments in χEFT have extended the applicability of these theoretical models up to 1.5 times the nuclear saturation density. This provides a robust low-density constraint which is essential for bridging astrophysical observations and high-density EOS predictions.

Methodology

The paper employs Bayesian inference methodologies with two distinct high-density EOS parameterizations: a piecewise-polytropic (PP) model and a speed of sound (CS) model. These models are used to extrapolate beyond the densities reliably described by χEFT, providing a comprehensive toolset to interpret astrometric and X-ray data alongside theoretical nuclear physics constraints.

Results and Implications

1. Radius Constraints: The EOS models predict relatively narrow range of radii for a canonical 1.4 solar-mass neutron star, centered around 12 km. Specifically:
   • The PP model yields radii of $12.28^{+0.50}_{-0.76}$ km, while
   • The CS model yields radii of $12.01^{+0.56}_{-0.75}$ km.
2. Maximum Mass and EOS Characteristics: Both models estimate the maximum mass of neutron stars (often denoted $M_{\rm TOV}$) to be around 2.15 $M_\odot$ for PP and slightly lower for CS. This conclusion, notably in line with high mass measurements from radio pulsars, limits the stiffness of the EOS at high densities, which has profound implications for the presence of exotic states such as quark deconfinement in neutron stars.
3. Sensitivity to χEFT Transition Densities: The extension of the χEFT framework to $1.5 n_0$ significantly enhances the resolution of EOS constraints, highlighting the utility of improved theoretical models in pinning down NS core properties.

Future Directions

The paper pioneers the integration of multimessenger astrophysical data with advanced nuclear theory inputs to explore the dense matter EOS. As NICER continues to accrue data and advancements in χEFT calculations refine low-density inputs, further constraints on the neutron star EOS will emerge. The anticipated inclusion of new GW and radio timing data can further refine these constraints, potentially illuminating exotic phases of matter within neutron stars, such as hyperons or deconfined quarks, pushing the frontiers of nuclear and astrophysics forward significantly.

This research represents a robust step towards converging diverse scientific methodologies into a comprehensive understanding of one of the cosmos's most intriguing constituents—the neutron star.