Astronomers Discover Lowest-Mass Double Neutron Star System, Confirming General Relativity

Astronomers have announced the discovery of the lowest-mass double neutron star (DNS) system identified to date, a finding that provides crucial new evidence supporting Einstein's theory of general relativity. The system, designated PSR J1856–0039, was discovered using China's Five-hundred-meter Aperture Spherical radio Telescope (FAST).

Neutron stars are the incredibly dense remnants of massive stars that exploded at the end of their lives. Some of these, known as pulsars, emit beams of radio waves as they spin, appearing to pulse as these beams sweep past Earth. According to Einstein's theory, orbiting neutron stars should generate gravitational waves, causing a gradual loss of energy that draws them closer and shortens their orbital period.

Researchers from the Chinese Academy of Sciences, the State Key Laboratory of Radio Astronomy and Technology in Beijing, and other Chinese academic institutions embarked on a study to test these predictions. Their findings, published in *Physical Review Letters*, confirm that PSR J1856–0039 exhibits an unusually low combined mass and its shrinking orbit precisely aligns with the predictions of general relativity.

JinLin Han, a co-author of the paper, informed Phys.org that FAST's pulsar survey has uncovered approximately 900 pulsars, with PSR J1856−0039 being a particularly significant discovery. First detected on May 4, 2020, this DNS system possesses an orbital period of 2.36 hours, making it the second shortest known among confirmed DNS systems. Its remarkably compact orbit and measurable relativistic effects render it an invaluable system for testing the fundamental theory of general relativity.

Over five years of long-term timing monitoring, the team precisely measured three post-Keplerian orbital parameters. These measurements allowed them to determine the individual masses of both neutron stars, confirming that their combined mass is the lowest ever recorded for any DNS system. The data, collected from 17 observation sessions between 2020 and 2025 at FAST in China's Dawodang depression, involved extracting 253 measurements of pulse arrival times and employing advanced timing analysis software like Tempo2 and PSRCHIVE. The changes in pulse arrival times revealed a shortening orbital period, a gradual turning of the orbit's closest point, and a slight timing shift known as the Einstein delay.

The researchers used these observations to estimate the merger time of the two neutron stars, projecting it to occur in approximately 82 million years. This system also ranks second among confirmed DNS systems in the strength of its relativistic effects, making it a prime candidate for further tests of general relativity, including the potential detection of the Lense–Thirring (frame-dragging) effect. Its lowest total mass suggests its merger will likely produce a massive neutron star rather than a black hole, offering unique insights into the equation of state of ultradense matter. Han and colleagues plan to continue tracking the system to understand how a neutron star's spin affects its orbit and to potentially achieve the first determination of the state of matter inside neutron stars.

What to watch: Further observations of PSR J1856–0039 for insights into ultradense matter and general relativity.

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