Detection of relativistic orbital deformation from improved timing of PSR J1757 - 1854

PSR J1757 - 1854, a 21.5,ms pulsar, is a highly relativistic double neutron star (DNS) system in a tight eccentric (e = 0.61) 4.4,hr orbit. With extremely large gravitational wave luminosity and one of the fastest orbital decay rates of any known DNS system, it is ideal for testing general relativity (GR) in the strong-field regime. Here we present results from a high-precision timing campaign combining archival data from the Murriyang telescope and Green Bank Telescope (GBT) with new high-sensitivity observations from the MeerKAT radio telescope and additional observations from the GBT. The extended baseline and superior sensitivity of MeerKAT have yielded substantial improvements to previously measured post-Keplerian parameters by a factor of around about2 or more. We report the first detection of the relativistic angular deformation, delta theta in this system, making PSR J1757 - 1854 only the third DNS system for which delta theta has been measured, achieved here in just 9 yrs compared to the decades of timing required for both the double pulsar and the Hulse-Taylor binary. We demonstrate how delta theta can be used to constrain the spin-orbit geometry of the system, ruling out two of the four geometric solutions previously identified, while remaining consistent with GR. We also evaluate higher-order contributions to the periastron advance, including the second post-Newtonian correction and the Lense-Thirring term, and show that these have a measurable systematic effect on the inferred total system mass. The observed orbital period derivative, b remains consistent with the GR prediction for gravitational-wave damping across a wide range of plausible distances.