The system, designated PSR J1856-0039, lies between 4,200 and 6,500 light-years from Earth. It was detected in 2020 during a survey of the Milky Way by the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in China, the world's largest single-dish radio telescope. Co-author JinLin Han of the National Astronomical Observatories of the Chinese Academy of Sciences said his team had discovered about 900 pulsars with FAST, but this one was "very extraordinary" after follow-up observations.
The pulsar spins about 43 times per second, and the two neutron stars are separated by roughly twice the Earth-Moon distance. Their short orbital period, the second shortest known, allows strong relativistic effects to be observed. The team made 17 observations between 2020 and 2025, sometimes covering an entire orbit, and measured pulse arrival times to extract orbital parameters.
Three effects were detected. First, the orbit is shrinking as gravitational waves carry away energy, bringing the stars about 2.7 metres closer each year. Second, the orbit's orientation rotates by 17.6 degrees per year, roughly 150,000 times faster than the relativistic drift of Mercury's orbit. Third, time dilation from gravity and speed causes the pulsar's clock to vary along its oval orbit, shifting pulse timings by up to about half a millisecond.
From these measurements, the team determined the masses: the pulsar at about 1.3 solar masses and its companion at 1.19 solar masses, making them the lightest known neutron star pair. The total mass is about 2.49 solar masses. The stars are expected to merge in about 82 million years, likely forming a heavier neutron star or possibly a black hole.
Han noted that any two of the three effects suffice to calculate the masses; the third then serves as an independent check. The fact that all three point to the same masses confirms that general relativity describes the system's behaviour, with consistency around 1%. While the more famous double pulsar gives a more precise test at 0.01%, that measurement used 16 years of data, compared with about five for this one.
"Unfortunately, we could not beat this description of gravity," Han said, referring to physicists' hopes of finding cracks in Einstein's theory that might point to new physics. The team now aims to detect "frame dragging," a twisting of space-time by the spinning neutron star, which would reveal the star's moment of inertia and provide a novel constraint on how matter behaves at crushing densities inside neutron stars. Monitoring will likely continue for decades.