In the past five years, some inspiring progress has been made in astrophysical observations of neutron stars. A recent study examined the equation of state (EOS) of the neutron star (NS).

Massive neutron stars likely have a strange 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 drew on neutron star observations and theory of quantum chromodynamics. The observation of this unusual core provides a rare opportunity to investigate the equation of the state of dense matter, specifically the change from hadronic to quark matter.

For the study, scientists analyzed data on the mass and radius of neutron stars, gravitational waves from binary neutron star mergers, and theoretical limitations of quantum chromodynamics calculations. The structure of the dense matter equation of state was carefully investigated using their combined knowledge and newly developed statistical methodology, the Bayesian non-parametric method.

This method allowed them to constrain the EOSs and study the speed of sound properties of neutron star matter. They found that a peak structure in the speed of sound of the equation of state, positioned at a density lower than the center density of the most massive neutron star, often appeared posterior. This non-monotonic behavior implied that the state differs from pure hadronic matter.

In addition, based on a posterior that meets both empirical and theoretical constraints, they discovered evidence for the existence of an exotic core in neutron stars more massive than 0.98 times the most massive.

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 (>1 km) is plausible..”

Magazine reference:

  1. Ming Zhe han, Yong Jia Huang et al. Plausible presence of the new state in neutron stars with masses greater than 0.98 MTOV. Science bulletin. DOI: 10.1016/j.scib.2023.04.007