The Relativistic Spin Precession in the Compact Double Neutron Star System PSR J1946+2052

We observe systematic profile changes in the visible pulsar of the compact double neutron star system PSR J1946+2052 using observations with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The interpulse of PSR J1946+2052 changed from a single-peak to a double-peak shape from 2018-2021. We attribute this evolution as the result of the relativistic spin precession of the pulsar. With the high sensitivity of FAST, we also measure significant polarization for the first time, allowing us to model this with the precessional rotating vector model. Assuming, to the first order, a circular hollow-cone-like emission beam pattern and taking the validity of general relativity (GR), we derive the binary's orbital inclination angle ( 63 degrees - 3 degrees + 5 degrees ) and pulsar's spin geometry. The pulsar's spin vector and the orbital angular momentum vector are found to be only slightly misaligned ( 0. degrees 21 - 0. degrees 10 + 0. degrees 28 ). The quoted uncertainties do not reflect the systematic uncertainties introduced by our model assumptions. By simulating future observations of profile and polarization evolution, we estimate that we could constrain the precession rate within a 43% uncertainty in 9 yr. Hence, we suggest that the system's profile evolution could be combined with precise pulsar timing to test GR in the future.