Recently, relying on the ultra-high detection sensitivity of China's Sky Eye FAST, the FAST galactic plane pulsar survey team led by researcher Han Jinlin of the National Astronomical Observatory made an important discovery and captured a double neutron star system PSRJ1856-0039 in a tight orbit. This system is a binary neutron star system with an extremely short orbital period and the lightest total mass currently known. Multiple observation results have accurately verified the general theory of relativity, providing a key new goal for exploring extreme astrophysics, the nature of gravity, and the origin of elements in the universe. The relevant research results were published in Physical Review Letters (PRL) on September 15, 2026, and were selected by the journal editor as a key recommendation result.
Neutron stars are extremely dense celestial body remnants formed after the death and collapse of massive stars. They operate with precise and stable rules and are known as the "precision clock of the universe." The binary neutron star system consists of two neutron stars orbiting each other. It is an extremely rare special celestial body system in the universe. Only 30 cases have been discovered by humans so far. This type of celestial system was formed after two supernova explosions and will merge and evolve through gravitational wave radiation in the future. It is an important birth source of heavy elements such as gold and platinum in the universe. It is also an ideal natural laboratory for studying extremely high-density matter states, testing general relativity, and analyzing the mechanism of supernova explosions. Among them, short-period double neutron stars have stronger relativistic effects and shorter merger times, and have particularly outstanding scientific research value. They have always been the core scientific goal of pulsar surveys by international large-scale radio telescopes.
As the world's most sensitive single-aperture radio telescope, FAST is equipped with a high-performance L-band 19-beam cooling receiver and has extremely strong pulsar detection capabilities. In this study, the team relied on a self-developed snapshot observation mode to carry out a large-scale systematic search for galactic plane pulsars, and so far have successfully discovered about 900 new pulsars. Through continuous follow-up precise observations, the team identified multiple cases of special celestial systems, and PSRJ1856-0039 was one of the breakthrough discoveries.
Observational data show that the orbital period of this binary neutron star system is only 2.36 hours, ranking second among the known binary neutron star systems. The extremely short orbital period means that the distance between the two neutron stars is extremely small and the orbits of the binary stars orbiting each other are extremely dense. It is one of the binary neutron star systems with the most significant relativistic effects currently known. At the same time, this system has refreshed the lower limit of the mass of double neutron stars known to mankind. The overall mass is only 2.488 times the mass of the sun, making it the lightest double neutron star combination ever discovered. Among them, the mass of the visible pulsar is about 1.30 times the mass of the sun, and the mass of the companion neutron star is about 1.19 times the mass of the sun. This companion star ranks among the lightest known neutron stars in the world, approaching the lower theoretical mass limit of neutron stars, providing strong constraints for the precise analysis of the physical mechanism of supernova explosions.
Thanks to the extremely dense orbit, the system has an extremely strong gravitational environment, and the research team successfully observed a number of clear relativistic effects. They accurately detected the periapsis precession of the elliptical orbit, the red shift of the photon frequency caused by the strong gravitational field, the time dilation effect caused by high-speed motion, as well as the system's energy loss due to continuous radiation of gravitational waves and the slow decay of the orbital period. Data calculations show that the ratio of the orbital period attenuation value measured this time to the value predicted by general relativity is 1.009±0.014, confirming the correctness of general relativity.
According to the team's model deduction, the system will merge in about 82 million years, and will eventually form a more massive neutron star with a high probability. This special evolutionary path plays an important role in solving the problem of the equation of state inside the neutron star and exploring the origin of heavy elements such as gold and platinum in the universe.
The system also has the capability for subsequent scientific breakthroughs. Its extremely small orbital inclination and extremely short orbital period create excellent conditions for detecting the reference frame drag effect produced when the pulsar rotates. Currently, there are only one or two examples of binary neutron star systems with the potential to detect this effect. The reference frame drag effect is directly related to the rotational inertia of the neutron star. Subsequently, relying on FAST's continuous high-precision observations, it is expected to accurately measure the rotational inertia of the neutron star. Combined with the accurately measured neutron star mass data, long-term observation and research can provide key constraints for cutting-edge scientific issues such as revealing the internal material state of neutron stars and exploring the basic characteristics of space-time gravity.
This research was supported by the key R&D projects of the Ministry of Science and Technology and the institutionalization project of the Chinese Academy of Sciences. To learn more about the "FAST Galactic Plane Pulsar Survey" project and related data, please visit the project website: http://zmtt.bao.ac.cn/GPPS/.
