The role of PSR J0614-3329 in defining the high-density matter at Neutron star cores

TL;DR

This work addresses constraining high-density neutron-star matter by combining NICER and GW data within a model-agnostic, speed-of-sound $c_s^2(\mu)$ framework, paying particular attention to PSR J0614-3329. It implements three EoS classes (monotonic, non-monotonic, discontinuous) and infers ten parameters via Bayesian nested sampling, integrating GW170817, NICER, and CET constraints. The results indicate a preference for smoother EoS with phase transitions occurring at higher densities, a lowered maximum mass $M_{\rm TOV}$, and a decoupling between maximum-mass and maximum-compactness sequences, with low-density EoS tightly constrained and high-density behavior remaining uncertain. PSR J0614-3329 exerts an effect commensurate with GW170817 in shaping the M–R and $\Lambda$–$M$ landscapes, and low-density PT in the discontinuous class is disfavored by the data. Overall, the joint NICER+GW analysis demonstrates substantial progress in decoding dense-matter phase structure and sets the stage for tighter future constraints as observations improve.

Abstract

In this work, we investigate how astrophysical observations from NICER and GW data constrain the matter properties at high densities, with a primary focus on the recent PSR J0614-3329 data. We have constructed three distinct classes of an ensemble of agnostic equation of state by speed of sound parametrisation. Bayesian inference is then employed to constrain the EoS construction parameters-namely, the squared speed of sound and chemical potential at each interpolation segment-using the observational data. Both the NICER and GW constraints hint towards a smoother EoS where PT occurs late, significantly reducing the maximum mass of NS. Also, the maximum-mass and maximum-compact sequences are distinctly different, as the former allows for the maximally stiff EoS to maximise the star mass. In contrast, the latter prefers a softer low-density and stiffer high-density EoS to maximise the compactness. The Bayesian analysis demonstrates that the observational bounds are effective in significantly constraining the low-density region of the equation of state. It is also seen that the astrophysical bound prefers the phase transition in the intermediate-density range and also prefers a small density jump for a discontinuous equation of state.

Figures (10)

• Figure 1: The 90% CI of M-R posterior distribution for different combinations of observational constraints for the non-monotonic set (left), monotonic set (middle), and discontinuous (right) is shown. Different colour contours signify different observational constraints.
• Figure 2: Contours of the speed of sound for the monotonous (left), non-monotonous (middle) and discontinuous (right) classes under observational constraints are plotted in the figure. The monotonous sound speed exhibits an initial steep rise, followed by a gradual increase. The sound speed decreases beyond the NS densities. However, a peak in the non-monotonous contours indicates a phase change, while in the discontinuous case, the transition is marked by a sudden drop of the sound speed to zero.
• Figure 3: mass–radius (M–R) contours from the speed of sound, highlighting phase changes, with the onset of different phases represented by distinct shaded regions as indicated in the legend box. The left panel corresponds to the non-monotonic class, while the right panel shows the discontinuous class. The upper panels present results without imposing the PSR J0614-3329 constraint, whereas the lower panels include this observational constraint.
• Figure 4: The M-R contour for the monotonic, non-monotonic and discontinuous EoS set showing the entire contour (solid purple contour) and the 90 % confidence contour (dashed purple contour) is plotted in the figure. The red and blue scatter points represent the maximum compactness and maximum mass, respectively, along with their corresponding M-R sequences.
• Figure 5: The entire span of the corresponding set of EoS (purple shaded region) for the three classes of EoS is shown in the different panels of the plot. The EoS corresponding to the maximum compactness and maximum mass is shown explicitly with red and blue curves, respectively.