The interiors of neutron stars contain matter at the highest densities realized in our Universe. Interestingly, theoretical studies of dense matter, in combination with the existence of two-solar-mass neutron stars, indicate that the speed of sound $c_s$ has to increase to values well above the conformal limit ($c_s^2=1/3$) before decreasing again at higher densities. The decrease could be explained by either a strong first-order phase transition or a crossover transition from hadronic to quark matter. The latter scenario leads to a pronounced peak in the speed of sound, reaching values above the conformal limit, naturally explaining the inferred behavior. In this work, we use the nuclear-physics multimessenger astrophysics (NMMA) framework to compare predictions of the quarkyonic matter model with astrophysical observations of neutron stars, with the goal of constraining model parameters. Assuming quarkyonic matter to be realized within neutron stars, we find that there can be a significant amount of quarks inside the cores of neutron stars with masses in the two-solar-mass range, amounting to up to $\approx 0.13M_{\odot }$, contributing $\approx 5.9\%$ of the total mass. Furthermore, for the quarkyonic matter model investigated here, the radius of a $1.4M_{\odot }$ neutron star would be $13.{44}_{-1.54}^{+1.69}(13.{54}_{-1.04}^{+1.02})\mathrm{km}$, at $95\%$ credibility, without (with) the inclusion of AT2017gfo.

• Received 30 August 2023
• Accepted 13 December 2023

DOI:https://doi.org/10.1103/PhysRevC.109.025807