Those arrival times revealed three distinct effects predicted by general relativity. The first is orbital decay: according to Einstein, orbiting neutron stars should emit gravitational waves — ripples in spacetime itself — and the resulting energy loss should gradually pull the pair closer together, shortening the time each orbit takes. The second is periastron advance, the slow turning of the orbit's closest point. The third is the Einstein delay, a slight timing shift caused by relativistic effects.

"This precision enabled robust measurement of three post-Keplerian parameters — the orbital period derivative, the rate of periastron advance and the Einstein delay — within the Damour–Deruelle general relativistic framework implemented in Tempo2," said Han, referring to the widely used pulsar timing software his team relied on.

From these three measurements, the researchers calculated the individual masses of both neutron stars. Their combined mass came out as the lowest yet measured for any double neutron star system. The team also compared the observed shrinkage of the orbit with the rate general relativity predicts. The two agree.

Why it matters

The system's unusual properties have consequences beyond testing relativity. Because its total mass is so low, the eventual merger of the two stars will likely produce a massive neutron star rather than a black hole. That outcome would offer astronomers a rare window into the equation of state of ultradense matter — the physical rule describing how matter behaves at the extreme densities found inside neutron stars, which cannot be reproduced in any laboratory on Earth.

The researchers estimate that the merger itself lies roughly 82 million years in the future. The remnant could be a stable neutron star, though it might instead collapse into a black hole after its rotation slows.

Han points to another prize within reach. "This system possesses the highest potential for detecting the Lense–Thirring effect due to its compact orbit, favorable inclination and exceptional timing precision," he said. The Lense–Thirring effect, also called frame-dragging, is a prediction of general relativity in which a spinning object drags spacetime around with it, slightly altering the motion of anything orbiting nearby.

Detecting that effect in this system would let astronomers measure the neutron star's moment of inertia — a quantity describing how its mass is distributed. "That could lead to the first determination of the state of matter inside neutron stars," Han explained.

A long road ahead

The faintness of the source makes the work demanding. Its mean flux density, a measure of radio brightness, is approximately 0.1 millijansky, and it varies from session to session. FAST's exceptional sensitivity allows high signal-to-noise detection of such a weak signal, and the team rigorously calibrated the data for instrumental effects using PSRCHIVE software before building their timing model.

The project is far from over. "We will continue long-term timing of this valuable system to better understand its relativistic effects and measure more post-Keplerian parameters," said Han. "It will be difficult, long-term work, probably lasting 10 years, and will be done by my postdoc Dr. Zonglin Yang."

The team also plans to use the system to investigate how a neutron star's spin affects its orbit, and what that subtle effect could reveal about the matter packed inside these stars.

As with any single-system study, the current results rest on five years of timing data, and the most ambitious goals — such as measuring the moment of inertia — remain unconfirmed until further observations come in. Still, PSR J1856–0039 has already earned its place as a benchmark system: the lightest double neutron star pair on record, shrinking exactly as Einstein said it should.