Chinese telescope detects lightest-known neutron star pair locked in decaying orbit

Astronomers in China have discovered the lightest pair of neutron stars ever found locked in orbit around each other, a compact and rapidly decaying system that gives physicists a new and unusually sensitive laboratory for testing Einstein's general theory of relativity. The system, catalogued as PSR J1856−0039, was detected with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the world's largest single-dish radio telescope, which sits in a natural karst basin in China's Guizhou province.

The findings, from a team led by JinLin Han of the Chinese Academy of Sciences' National Astronomical Observatories and the State Key Laboratory of Radio Astronomy and Technology in Beijing, were published Thursday in Physical Review Letters. The paper describes five years of precision timing that pinned down the two stars' masses, their orbital geometry and the rate at which the pair is spiraling toward a future collision.

A record for the lightest known pair

Double neutron star systems form when both members of a binary pair of massive stars explode as supernovae without disrupting the orbit, leaving two ultradense stellar corpses still gravitationally bound to each other. Only a couple of dozen such systems are known in the Milky Way, and PSR J1856−0039 is now the lightest of them by combined mass.

According to the National Astronomical Observatories' announcement and the published paper, the visible pulsar carries a mass of about 1.304 solar masses and spins once every 23.4 milliseconds, while its unseen companion neutron star weighs in at roughly 1.185 solar masses — among the lowest neutron-star masses ever measured. Together the pair totals just 2.488 solar masses, edging below every previously confirmed double neutron star binary.

The two stars circle each other once every 2.36 hours, the second-shortest orbital period known for a confirmed double neutron star system, on an eccentric path that brings them close enough together that relativistic effects become measurable within a few years of observation rather than decades.

The numbers behind the discovery

Researchers first spotted the pulsar on May 4, 2020, during FAST's Galactic Plane Pulsar Snapshot survey, and followed up with 17 dedicated observing sessions through 2025, gathering 253 precise pulse arrival-time measurements. That timing baseline let the team measure three independent relativistic effects predicted by general relativity:

• A periastron advance — the same kind of orbital precession Einstein used to explain an anomaly in Mercury's orbit — of 17.5859 degrees per year, among the fastest ever measured in a binary pulsar.
• An Einstein delay of 0.445 milliseconds, capturing the combined effects of gravitational redshift and time dilation as the pulsar swings through its orbit.
• An orbital decay rate matching the energy loss expected from gravitational-wave emission to within about 1 percent of general relativity's prediction.

That last measurement means the system is losing energy to gravitational waves exactly as Einstein's equations predict, slowly shrinking the orbit. Researchers calculate the two neutron stars will spiral together and merge in roughly 82 million years, far too slow to be observed directly but fast enough, on cosmic timescales, to make the system a long-term target for monitoring.

Why such a light pair matters

The discovery matters to two overlapping communities of scientists. For specialists in gravitational physics, a compact, eccentric orbit like this one is valuable because relativistic effects scale with how close and fast the stars move past each other; the tighter the orbit, the more precisely general relativity's predictions can be checked against reality. Han's team notes in the paper that continued timing of PSR J1856−0039 could eventually let them detect the Lense-Thirring effect, also known as frame dragging, in which a spinning massive object twists the fabric of spacetime around it — an effect so subtle it has been measured directly only a handful of times.

For astrophysicists who study how matter behaves at nuclear densities, the unusually low mass of the companion star is just as important. Theoretical models of dense matter, known as equations of state, must be able to produce a stable neutron star as light as 1.185 solar masses, which narrows the range of physically plausible models. Some researchers suspect such low-mass neutron stars form through a gentler supernova mechanism, called electron-capture collapse, rather than the more violent core-collapse explosions that produce heavier neutron stars.

The result also feeds into the broader effort to understand neutron-star mergers, the cosmic events that produce short gamma-ray bursts and forge heavy elements like gold and platinum, famously observed in the 2017 LIGO-Virgo detection of merging neutron stars known as GW170817. Knowing the true range of masses and orbital configurations among double neutron star systems in our own galaxy helps researchers interpret the gravitational-wave signals that detectors like LIGO, Virgo and KAGRA are expected to pick up from similar, more distant mergers in the coming years.

What the researchers are saying

Han's team has used FAST's exceptional sensitivity to conduct a large-scale survey of the galactic plane, turning up hundreds of new pulsars since the telescope began operating. "We are conducting a pulsar survey using FAST and have discovered approximately 900 pulsars to date," Han said, according to a summary of the findings. "Among these, PSR J1856−0039 stands out as a particularly significant discovery."

"This system possesses the highest potential for detecting the Lense–Thirring (frame-dragging) effect due to its compact orbit," Han said.

Han added that the team intends to keep watching the pair. "We will continue long-term timing of this valuable system to better understand its relativistic effects," he said. The group's preprint, first posted in July and revised in September ahead of publication, lays out the full set of orbital and mass measurements for other astronomers to scrutinize and build on.

What happens next

FAST's pulsar survey is continuing to scan the galactic plane, and astronomers expect more double neutron star systems to turn up as the telescope's snapshot survey mode covers more sky. Each new system adds a data point to the still-small census of these objects, helping refine statistical predictions about how often and in what configurations neutron stars merge across the universe.

For PSR J1856−0039 itself, the next steps are patience and precision: continued timing campaigns over the coming years should sharpen the measurement of the Lense-Thirring effect and could reveal still more relativistic signatures in the pulsar's radio pulses. With 82 million years before the two stars finally merge, there is no shortage of time to keep watching — though the data already in hand have given physicists one of their better tools yet for probing gravity at its most extreme, and the physics of matter denser than anything that can be created on Earth.