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 orb=-1.284 plus or minus0.019 times10-12 s s-1, periastron advance omega=17.5859 plus or minus0.0007 deg yr-1, and Einstein delay gamma=0.445 plus or minus0.011 ms. This DNS system has a low orbital inclination of i=133.2 plus or minus1.1 and the lowest total mass of any known DNS, M tot=2.48841 plus or minus0.00015 M, with a determined pulsar mass of 1.304 plus or minus0.022 M and a companion mass of 1.185 plus or minus0.022 M, one of the lowest neutron-star masses. The observed orbital decay due to gravitational-wave emission GW orb,obs and the orbital decay predicted by general relativity GW orb,pred are consistent at a level of GW orb,obs/ GW orb,pred= 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.