Astronomers Found the Lowest-Mass Double Neutron Star Pair Yet

Astronomers working with China's Five-hundred-meter Aperture Spherical Radio Telescope, better known as FAST, have measured the lowest-mass double neutron star system ever found. The pair, catalogued as PSR J1856–0039, carries a combined weight of roughly 2.49 solar masses, a new record for lightness among the handful of confirmed double neutron star binaries in the sky.

The system first showed up in FAST data on May 4, 2020, as a pulsing radio source spinning once every 23.4 milliseconds. Follow-up timing turned it into one of the richest gravity laboratories astronomers have. The research team, led by Zonglin Yang and JinLin Han of the Chinese Academy of Sciences, collected 253 pulse arrival-time measurements across 17 observing sessions between 2020 and 2025, building a timing model precise enough to weigh each star individually. That precision makes it the lowest-mass double neutron star on record and a standout target for gravity tests.

What emerged is a remarkably tight binary. The two stars circle each other every 2.36 hours, making it the second-shortest orbit of any confirmed double neutron star system. The pulsar tips the scales at about 1.304 solar masses, while its companion weighs roughly 1.185 solar masses, ranking among the lightest neutron stars ever detected.

The orbit is slightly eccentric rather than a perfect circle, and the pair is tilted at about 133 degrees to our line of sight, geometry that the timing model pinned down alongside the masses. All of it was measured with FAST, the giant dish set in the Dawodang depression, a natural basin in southwestern China whose 500-meter aperture can pick up faint pulses that smaller telescopes miss. That sensitivity, the team notes, is what turned a dim flicker into the lowest-mass double neutron star known.

A stress test for Einstein

Because the orbit is so compact, the pair's gravitational dance betrays itself in the arrival times of the radio pulses. The team measured three effects predicted by general relativity: the slow shrinking of the orbital period, the gradual rotation of the orbit's closest point, and a subtle timing shift called the Einstein delay. According to the researchers, the observed orbital decay matches the amount predicted by Einstein's theory to within about one percent, a tight agreement reported in the study published in Physical Review Letters.

That precision makes the lowest-mass double neutron star unusually valuable. It ranks second among confirmed double neutron star systems in the strength of its relativistic effects, and the authors write that it offers the best current chance of detecting frame dragging, a subtle effect where the spinning neutron star twists spacetime itself. Catching that signal would open a direct line into the matter packed inside neutron stars, the densest known form of visible matter in the universe.

What a light merger would look like

The two stars are slowly spiraling toward each other and should merge in about 82 million years. Because the combined mass is so low, the lowest-mass double neutron star will probably forge a single massive neutron star instead of a black hole when they collide, a result that would pin down the equation of state of ultradense matter, the physics describing how matter behaves when it is squeezed harder than anywhere else in nature.

Han says the team plans to keep timing the system for another decade or so, with postdoctoral researcher Zonglin Yang leading the effort, in hopes of measuring the frame-dragging signal and eventually the neutron star's moment of inertia. Either measurement would mark the first of its kind.

The discovery also reflects how productive FAST has become. As Han told Phys.org, the pulsar survey behind the find has uncovered roughly 900 pulsars to date, and the lowest-mass double neutron star of them all stood out immediately. For gravitational-wave astronomy, every new double neutron star is a preview of the collisions that detectors like LIGO will one day hear, and the lightest one yet gives theorists a fresh case to model. More details appear in the paper's arXiv preprint, and Phys.org reported the find with additional background from the team.