Chinese scientists discover lightest, shortest-orbit binary neutron star system

Photo shows the Five-hundred-meter Aperture Spherical Radio Telescope. (Photo courtesy of the National Astronomical Observatories, Chinese Academy of Sciences)

A team led by Han Jinlin, a researcher at the National Astronomical Observatories of the Chinese Academy of Sciences, has made a major discovery using the ultra-high detection sensitivity of China's Five-hundred-meter Aperture Spherical Radio Telescope (FAST).

The team detected pulsar J1856−0039, part of a binary neutron star system in a compact orbit. This system boasts an extremely short orbital period and is the lowest-mass binary neutron star system known to date.

The finding, based on multiple precise observations, provides a stringent test of general relativity and offers a new target for studying extreme astrophysical phenomena, the nature of gravity, and the origins of chemical elements in the universe. The research was recently published in Physical Review Letters.

Neutron stars are extremely dense remnants formed when massive stars collapse at the end of their lives. Their remarkably precise and stable rotations have earned them the nickname "cosmic clocks."

A binary neutron star system consists of two neutron stars orbiting each other and is an exceptionally rare type of celestial system. Such systems form through two successive supernova explosions and eventually evolve toward a merger through the emission of gravitational waves.

They are thought to be important sources of heavy elements such as gold and platinum, and they serve as natural laboratories for studying matter at extreme densities, testing general relativity, and probing the mechanisms behind supernova explosions.

As the world's most sensitive single-dish radio telescope, FAST has exceptional capabilities for detecting pulsars. Using a snapshot observing mode independently developed by the team, researchers have conducted a large-scale, systematic search for pulsar systems along the Galactic plane and have discovered about 900 new pulsars to date. Continued precision follow-up observations have enabled the team to identify a number of unusual celestial systems, including pulsar J1856−0039.

Observations show that the system has an orbital period of just 2.36 hours, making it the second-shortest known orbital period among binary neutron star systems. Such a short orbital period means that the two neutron stars are separated by a very small distance and orbit each other in an extremely compact configuration. As a result, relativistic effects in this system are among the most pronounced of any known double neutron star system.

The system also sets a new lower limit for the total mass of a binary neutron star system. With a combined mass of just 2.488 times that of the Sun, it is the lightest binary neutron star system ever recorded.

The visible pulsar has a mass of about 1.30 solar masses, while its companion neutron star has a mass of about 1.19 solar masses. The companion ranks among the lightest neutron stars known worldwide and is close to the theoretical minimum mass, providing an opportunity to more precisely probe the physics of supernova explosions.

According to the team's modeling, the two neutron stars will merge in about 82 million years, most likely forming a more massive neutron star. This unusual evolutionary pathway is important for addressing questions about the equation of state of matter inside neutron stars and for exploring the origins of heavy elements such as gold and platinum in the universe.

Han said that, combined with precisely measured neutron star masses, long-term observations and studies of pulsar binary systems can provide key constraints for addressing frontier questions in science, including the nature of matter inside neutron stars and the fundamental properties of gravity in spacetime.