This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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." 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.
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