Astronomers discovered the lowest-mass double neutron star system using the FAST telescope in China. First detected on May 4, 2020, the system PSR J1856–0039 has a compact orbit of 2.36 hours. JinLin Han said, "We have determined the individual masses and find that their combined mass is the lowest yet measured."
Neutron stars, the extremely dense remains of massive stars that exploded at the end of their lives, are widely studied astrophysical objects. Some of these stars, known as pulsars, spin and send out beams of radio waves, making them appear to pulse as the beams sweep past Earth.
According to Einstein's theory of general relativity, orbiting neutron stars should emit ripples in spacetime known as gravitational waves. The resulting loss of energy should gradually draw pairs of neutron stars closer together, shortening the time they take to complete each orbit.
Researchers at the Chinese Academy of Sciences, the State Key Laboratory of Radio Astronomy and Technology in Beijing and other academic institutions in China set out to test this prediction by studying PSR J1856–0039, a double neutron star (DNS) system discovered using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China.
Their findings, published in a paper in Physical Review Letters, show that the system has an unusually low combined mass and that its shrinking orbit is consistent with the predictions of general relativity.
"We are conducting a pulsar survey using FAST and have discovered approximately 900 pulsars to date," JinLin Han, a co-author of the paper, told Phys.org. "Among these, PSR J1856−0039 stands out as a particularly significant discovery. It was first detected on May 4, 2020. Subsequent FAST observations revealed that it is a double neutron star (DNS) system with an orbital period of 2.36 hours—the second shortest known among confirmed DNS systems."
Tracking a pulsar with strong relativistic effects
PSR J1856−0039, first detected by FAST in 2020, has a remarkably compact orbit and measurable effects predicted by general relativity. These features make it a valuable system for testing the theory.
"Over the past five years, long-term timing monitoring has enabled precise measurement of three post-Keplerian orbital parameters," said Han. "From these, we have determined the individual masses of both neutron stars and find that their combined mass is the lowest yet measured for any DNS system. In this paper, we present these results and discuss their implications for neutron star formation and binary evolution."
FAST is a large radio telescope located in the Dawodang depression, a natural basin in southwestern China. Astronomers observed PSR J1856−0039 in 17 sessions between 2020 and 2025, extracting 253 measurements of pulse arrival times.
"FAST's exceptional sensitivity enables high signal-to-noise ratio (S/N) detection of this relatively faint source—its mean flux density is approximately 0.1 mJy, though it exhibits session-to-session variability," explained Han. "Timing analysis was performed using the widely adopted pulsar timing software Tempo2. Prior to timing modeling, we rigorously calibrated the FAST data for instrumental polarization and bandpass response using PSRCHIVE."
By measuring when the pulsar's radio pulses reached FAST, the team tracked the pulsar's rotation and its motion around its companion. Changes in the pulses' arrival times revealed three effects: the shortening of the orbital period, the gradual turning of the orbit's closest point and a slight timing shift known as the Einstein delay.
The researchers used these measurements alongside general relativity to estimate the mass of each star and compare the observed orbital shrinkage with the predicted rate. They also estimated when the stars are likely to merge.
"We iteratively refined both the data reduction pipeline and timing solution, culminating in a highly precise timing model," said Han. "This precision enabled robust measurement of three post-Keplerian parameters—the orbital period derivative dot(Porb), the rate of periastron advance (dot-Omega) and the Einstein delay (Gamma)—within the Damour–Deruelle general relativistic (DDGR) framework implemented in Tempo2."
Deepening the understanding of neutron star mergers
According to the researchers, PSR J1856−0039 ranks second among confirmed double neutron star systems in the strength of its relativistic effects. This makes it a promising system for further tests of general relativity.
"This system possesses the highest potential for detecting the Lense–Thirring (frame-dragging) effect due to its compact orbit, favorable inclination and exceptional timing precision," said Han. "It hosts the lowest total mass measured to date among all DNS systems—a property that implies its merger will likely produce a massive neutron star rather than a black hole, offering unique insight into the equation of state of ultradense matter."
The researchers estimate that the two neutron stars will merge in approximately 82 million years. The merger could produce a stable neutron star, though the remnant might instead collapse into a black hole after its rotation slows. Han and his colleagues plan to continue tracking the system to investigate how a neutron star's spin affects its orbit and what that effect could reveal about matter inside neutron stars.
"We will continue long-term timing of this valuable system to better understand its relativistic effects and measure more post-Keplerian parameters," added Han. "In particular, we hope to measure the Lense–Thirring precession effect and then the neutron star's moment of inertia, which could lead to the first determination of the state of matter inside neutron stars. It will be difficult, long-term work, probably lasting 10 years, and will be done by my postdoc Dr. Zonglin Yang."