FAST Finds the Lightest Known Pair of Neutron Stars

China’s FAST radio telescope has identified the lightest known double-neutron-star system, giving astronomers a rare new laboratory for testing gravity and studying how collapsed stars form. Designated PSR J1856-0039, the pair has a total mass 2.488 times that of the Sun and completes an orbit every 2.36 hours, the second-shortest period among known systems of its kind.

Neutron stars are exceptionally dense remnants of massive stars. Only about 30 double-neutron-star systems have been found. In this one, the visible pulsar is about 1.30 solar masses, while its companion is about 1.19 solar masses, placing the companion among the lightest neutron stars known and close to the theoretical lower limit. That could help scientists narrow down how supernova explosions create such remnants.

Precision timing revealed several effects predicted by general relativity, including orbital precession, gravitational redshift, time dilation and gradual orbital decay as the system loses energy through gravitational waves. The measured decay was 1.009 ± 0.014 times the predicted value, a close match.

The discovery emerged from a FAST survey that has found about 900 new pulsars. Models suggest the pair will merge in roughly 82 million years, probably forming a more massive neutron star. Continued observations could also reveal clues about the structure of matter at extreme densities.