Strangeon stars

The average density of pulsar like compact objects exceeds the saturation density of nuclear matter, and the structure and properties of their internal matter are closely related to supernova explosions, gamma ray bursts, fast radio bursts, massive compact stars, and even dark matter and cosmic ray detection. However, due to the non-perturbative nature of quantum chromodynamics, there is still significant uncertainty in our understanding of the composition and equation of state for dense stellar matter. And a deeper understanding on this topic has become an important goal in the era of multi-messenger astronomy. Based on the flavor symmetry of quarks and the strong coupling, this review proposes a new perspective on the nature of pulsars, i.e., it is made of strangeons (in analogous to nucleons, i.e., multiquark-state with three flavor symmetry). Due to the large mass of strangeons and the strong interactions among them, strangeon matter forms a crystal structure at low temperature, forming solid-state strangeon stars. We will briefly introduce several physical models that describe strangeon matter and the corresponding structures and properties of strangeon stars, including the equation of state, surface properties, mass-radius relations, glitches, binary mergers, and so on. We will also explore the observational properties of pulsar-like compact objects that support the strangeon star model.