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

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 P[over ˙]_{orb}=-1.284±0.019×10^{-12}  s s^{-1}, periastron advance ω[over ˙]=17.5859±0.0007  deg yr^{-1}, and Einstein delay γ=0.445±0.011  ms. This DNS system has a low orbital inclination of i=133.[over ∘]2±1.[over ∘]1 and the lowest total mass of any known DNS, M_{tot}=2.48841±0.00015M_{⊙}, with a determined pulsar mass of 1.304±0.022M_{⊙} and a companion mass of 1.185±0.022M_{⊙}, one of the lowest neutron-star masses. The observed orbital decay due to gravitational-wave emission P[over ˙]_{orb,obs}^{GW} and the orbital decay predicted by general relativity P[over ˙]_{orb,pred}^{GW} are consistent at a level of P[over ˙]_{orb,obs}^{GW}/P[over ˙]_{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.