Massive Neutron Star Has a Strange Heart--Purple Mountain Observatory

Massive Neutron Star Has a Strange Heart

At the end of a star's life, nuclear fusion ceases, and the resulting pressure is no longer sufficient to counteract the gravitational force. This collapse can lead to the formation of neutron stars, which are composed of the densest matter in the universe. However, the composition of neutron stars has been the subject of much controversy. Prof. FAN Yizhong and his team at the Purple Mountain Observatory (PMO) of the Chinese Academy of Sciences (CAS), have used observations of neutron stars and quantum chromodynamics theory to conclude that a strange quark matter core likely exists in massive neutron stars. Observing this exotic core provides a unique opportunity to explore the equation of state of dense matter, particularly the transition from hadronic to quark matter.

The team analyzed data on neutron star mass and radius, gravitational waves from binary neutron star mergers, and theoretical constraints from quantum chromodynamics calculations. Using a newly developed statistical method, they studied the structure of the dense matter equation of state in detail.

The team found that a peak structure in the sound speed of the equation of state commonly appears in the posterior, located at a density lower than the center density of the most massive neutron star. Such non-monotonic behavior suggests that the state deviates from pure hadronic matter. Additionally, they found support for the existence of an exotic core in neutron stars heavier than 0.98 times the most massive one, based on a posterior that satisfies both observational and theoretical constraints. 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 (see Fig.1).

This work has been published in Science Bulletin under the title "Plausible presence of new state in neutron stars with masses above 0.98M_TOV."

Fig.1 Mass (MEC) and radius (REC) of the exotic core versus the neutron star mass divided by MTOV, the maximum gravitational mass of the non-rotating neutron stars. For the neutron stars lighter than 0.92 MTOV, a sizable exotic core (strange heart) is unlikely. While for the neutron stars more massive than 0.98 MTOV, a strange heart is plausible. (Image by HAN Mingzhe et al, 2023 Sci. Bull.)