Astronomers discover the lowest-mass double neutron star system to date

Astronomers at the Chinese Academy of Sciences and other institutions discovered the lowest-mass double neutron star system, PSR J1856–0039, using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The system has an orbital period of 2.36 hours, making it the second shortest known DNS system. The combined mass of the two neutron stars is approximately 2.49 times the Sun's mass, making it the lightest known DNS system. The discovery was made through long-term timing monitoring and analysis of FAST's data, revealing measurable effects predicted by general relativity.

• "We iteratively refined both the data reduction pipeline and timing solution, culminating in a highly precise timing model," said JinLin 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."

The system's low mass implies that 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 merging could produce a stable neutron star, though the remnant might instead collapse into a black hole after its rotation slows. The study highlights the potential for further testing of general relativity using this system.

The discovery is significant because it provides a rare opportunity to study the relativistic effects of a low-mass double neutron star system. The findings contribute to understanding neutron star formation and binary evolution. The study was conducted over 17 sessions between 2020 and 2025, with 253 measurements of pulse arrival times. The research team used advanced timing analysis and software like Tempo2 to measure the pulsar's rotation and motion around its companion. The results offer valuable data for future research on neutron star mergers and the behavior of ultradense matter.

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