In the past five years, there has been some inspiring progress in astrophysical observations of Neutron stars. A recent study investigated the neutron star (NS) equation of state (EOS).
Massive neutron stars are likely to have a weird quark matter core, according to a study led by Prof. FAN Yizhong of the Purple Mountain Observatory (PMO) of the Chinese Academy of Sciences (CAS), which was based on observations of neutron stars and quantum chromodynamics theory. The observation of this unusual core offers a rare chance to investigate the equation of the state of dense matter, notably the change from hadronic to quark matter.
For the study, scientists analyzed data on neutron star mass and radius, gravitational waves from binary neutron star mergers, and theoretical constraints from quantum chromodynamics calculations. The structure of the dense matter equation of state was carefully examined using their combined knowledge and newly devised statistical methodology called the Bayesian nonparametric method.
This method allowed them to constrain the EOSs and study the sound speed properties of neutron star’s matter. They discovered that a peak structure in the sound speed of the equation of state, positioned at a density lower than the center density of the most massive neutron star, frequently appeared in the posterior. It was implied by this non-monotonic behavior that the state differs from pure hadronic matter.
Additionally, based on a posterior that complies with both empirical and theoretical constraints, they discovered evidence in favor of the existence of an exotic core in neutron stars heavier than 0.98 times the most massive one.
Scientists noted, “Quantitative analysis revealed that the state at the center of the most massive neutron star is softer than typical hadronic matter (even with hyperons), and a sizable exotic core (>1km) is plausible.”
- Ming Zhe han, Yong Jia Huang et al. Plausible presence of the new state in neutron stars with masses above 0.98 MTOV. Science Bulletin. DOI: 10.1016/j.scib.2023.04.007