FAST telescope identifies lowest-mass double neutron star system PSR J1856–0039

Astronomers at the Chinese Academy of Sciences and the State Key Laboratory of Radio Astronomy and Technology have discovered the lowest-mass double neutron star (DNS) system recorded to date. According to a report in Phys.org, the system, designated PSR J1856–0039, was first detected on May 4, 2020, using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China. The findings, published in Physical Review Letters, confirm that the system’s orbital shrinkage aligns with the predictions of Albert Einstein’s theory of general relativity regarding gravitational waves.

Co-author JinLin Han told Phys.org that PSR J1856−0039 features an orbital period of 2.36 hours, making it the second shortest known among confirmed DNS systems. Researchers conducted 17 observation sessions between 2020 and 2025, collecting 253 measurements of pulse arrival times. FAST's sensitivity allowed for the detection of the faint source, which has a mean flux density of approximately 0.1 mJy. The team utilized Tempo2 pulsar timing software and the Damour–Deruelle general relativistic framework to measure three post-Keplerian parameters: the orbital period derivative, the rate of periastron advance, and the Einstein delay.

The system is notable for its unusually low combined mass, which suggests that its eventual merger will likely result in a massive neutron star rather than a black hole. This provides a unique opportunity for scientists to study the equation of state of ultradense matter. The researchers estimate the two neutron stars will merge in approximately 82 million years. Currently, PSR J1856−0039 ranks second among confirmed DNS systems for the strength of its relativistic effects, making it a primary candidate for future tests of the theory of general relativity.

Future research will focus on measuring the Lense–Thirring (frame-dragging) effect and the neutron star’s moment of inertia. Han and his colleagues intend to continue long-term timing monitoring to investigate how a neutron star’s spin affects its orbit. Such measurements could lead to the first definitive determination of the state of matter located inside neutron stars. The discovery is part of a larger pulsar survey using FAST, which has identified approximately 900 pulsars to date, though PSR J1856−0039 remains one of the most significant finds due to its compact orbit and favorable inclination.

What to watch: Future measurements of the Lense–Thirring effect in PSR J1856−0039.

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