Observable Signatures of a Quarkyonic Phase in Neutron Stars

Abstract

Performing Bayesian inference on quarkyonic equation-of-state models for neutron star matter, we find they satisfy all current astrophysical observations, thus reinforcing the argument for the use of such neutron star matter equation-of-state models alongside traditional ones. To observationally differentiate between stars with and without a quarkyonic phase, we identify a novel observational signature: the slope of the mass-radius relation at some fixed mass in conjunction with the sound speed at the star's center. In this plane, we find quarkyonic stars in a region with high central sound speed and positive slope, that is distinct from purely nucleonic stars. High accuracy NS radii measurements facilitated by the next generation of detectors, coupled with ongoing studies of mapping astrophysical observables to microphysical properties like sound speed can be used for testing this signature. Our results indicate that a neutron star with these properties would be a strong evidence for existence of a quarkyonic phase or a similar crossover transition in its core.

Figures (7)

• Figure 1: Correlation corner-plot of the seven parameters constrained via Bayesian inference at $1$, $2$ and $3\sigma$ confidence levels. Posterior distribution histogram alongside constrained values of parameters are shown along the diagonal. Red, purple, and blue are used for sets $\alpha$, $\beta$ and $\gamma$, respectively.
• Figure 2: 1-dimensional posteriors at $90\%$ confidence interval of, (a) pressure on energy density grid, (b) radius on mass grid, and (c) dimensionless tidal deformability on mass grid, for sets $\alpha$, $\beta$, and $\gamma$ - represented in solid red boundary, dashed purple boundary, and dashed blue boundary (with blue dots inside), respectively. Bounds on EoS from Refs. Annala:2019pufAltiparmak:2022bke are shown in (a) using solid black lines and the salmon shaded region. Colored patches in (b) show the $2\sigma$ bounds from NICER observations of PSR J0030+0451 (tan) Riley:2019yda, PSR J0740+6620 (salmon) Riley:2021pdl, PSR J0437-4715 (teal) Choudhury:2024xbk and PSR J0614-3329 (violet) Mauviard:2025dmd, and the $90\%$ ($50\%$) bounds from GW170817 in light (dark) orange LIGOScientific:2018cki. The gray region is (c) is the $90\%$ confidence region the $M-\Lambda$ posterior from GW170817 noauthor_ligo-p1800115-v12_nodate, and the observed $90\%$ ($50\%$) values for GW170817 components are shown using the light (dark) red and purple regions LIGOScientific:2018cki.
• Figure 3: Slope of $R(M)$ curve evaluated at different masses plotted against the central sound speed of canonical NS, with data points colored according to presence (absence) of a quarkyonic phase in the canonical NS using magenta (green). The dashed red (green) line marks the $95$-percentile upper-bound on $\dv*{R}{M}$ for the quarkyonic (hadronic) NSs. The $1$ and $2\sigma$ regions for NJL models from Ref. Albino:2025puc is shown in blue and black, respectively.
• Figure 4: Similar to Fig. \ref{['fig:3_cs2_composition']}, but plotting the slope $\dv*{R}{M}$ against the radius of the canonical NS.
• Figure 5: Probability distributions of the log-likelihoods of the three result sets shown using red, purple and blue lines. The sub-figures display distributions when log-likelihoods are calculated using all available datasets (upper-left), common datasets for all result sets (upper-right), only PSR J0437-4715 (lower-left), and only PSR J0614-3329 (lower-right).